diff --git a/Changes b/Changes
index 6336e6c2d..6ecedd938 100644
--- a/Changes
+++ b/Changes
@@ -6,6 +6,8 @@ The contributors that suggested a given feature are shown in []. Thanks!
 
 ****  Fix MacOS portability, bug1232. [Jeff Bush]
 
+****  Detect MSB overflow when under VL_DEBUG, bug1238. [Junyi Xi]
+
 
 * Verilator 3.914 2017-10-14
 
diff --git a/include/verilated.cpp b/include/verilated.cpp
index 869520aa0..da56d1e6f 100644
--- a/include/verilated.cpp
+++ b/include/verilated.cpp
@@ -1488,6 +1488,14 @@ const char* Verilated::commandArgsPlusMatch(const char* prefixp) VL_MT_SAFE {
     return outstr;
 }
 
+void Verilated::overWidthError(const char* signame) VL_MT_SAFE {
+    // Slowpath - Called only when signal sets too high of a bit
+    std::string msg = (std::string("Testbench C set input '")
+                       + signame
+                       + "' to value that overflows what the signal's width can fit");
+    VL_FATAL_MT("unknown",0,"", msg.c_str());
+}
+
 void Verilated::quiesce() VL_MT_SAFE {
 #ifdef VL_THREADED
     // Wait until all threads under this evaluation are quiet
diff --git a/include/verilated.h b/include/verilated.h
index 96d1ccdcd..a39eab78c 100644
--- a/include/verilated.h
+++ b/include/verilated.h
@@ -429,9 +429,14 @@ public:
     // METHODS - INTERNAL USE ONLY (but public due to what uses it)
     // Internal: Create a new module name by concatenating two strings
     static const char* catName(const char* n1, const char* n2); // Returns static data
+
+    // Internal: Throw signal assertion
+    static void overWidthError(const char* signame) VL_MT_SAFE;
+
     // Internal: Find scope
     static const VerilatedScope* scopeFind(const char* namep) VL_MT_SAFE;
     static const VerilatedScopeNameMap* scopeNameMap() VL_MT_SAFE;
+
     // Internal: Get and set DPI context
     static const VerilatedScope* dpiScope() VL_MT_SAFE { return t_s.t_dpiScopep; }
     static void dpiScope(const VerilatedScope* scopep) VL_MT_SAFE { t_s.t_dpiScopep=scopep; }
@@ -442,6 +447,7 @@ public:
     static const char* dpiFilenamep() VL_MT_SAFE { return t_s.t_dpiFilename; }
     static int dpiLineno() VL_MT_SAFE { return t_s.t_dpiLineno; }
     static int exportFuncNum(const char* namep) VL_MT_SAFE;
+
     static size_t serializedSize() VL_PURE { return sizeof(s_s); }
     static void* serializedPtr() VL_MT_UNSAFE { return &s_s; } // Unsafe, for Serialize only
 #ifdef VL_THREADED
diff --git a/src/V3AstNodes.h b/src/V3AstNodes.h
index cc1414aec..a5e322fbe 100644
--- a/src/V3AstNodes.h
+++ b/src/V3AstNodes.h
@@ -2878,7 +2878,7 @@ private:
     bool	m_unique0Pragma;	// unique0 case
     bool	m_priorityPragma;	// priority case
 public:
-    AstIf(FileLine* fileline, AstNode* condp, AstNode* ifsp, AstNode* elsesp)
+    AstIf(FileLine* fileline, AstNode* condp, AstNode* ifsp, AstNode* elsesp=NULL)
 	: AstNodeIf(fileline, condp, ifsp, elsesp) {
 	m_uniquePragma=false; m_unique0Pragma=false; m_priorityPragma=false;
     }
@@ -5153,6 +5153,7 @@ private:
     string	m_cname;		// C name, for dpiExports
     string	m_rtnType;		// void, bool, or other return type
     string	m_argTypes;
+    string	m_ifdef;		// #ifdef symbol around this function
     bool	m_dontCombine:1;	// V3Combine shouldn't compare this func tree, it's special
     bool	m_skipDecl:1;		// Don't declare it
     bool	m_declPrivate:1;	// Declare it private
@@ -5225,6 +5226,8 @@ public:
     void	funcPublic(bool flag) { m_funcPublic = flag; }
     void	argTypes(const string& str) { m_argTypes = str; }
     string	argTypes() const { return m_argTypes; }
+    void	ifdef(const string& str) { m_ifdef = str; }
+    string	ifdef() const { return m_ifdef; }
     void	funcType(AstCFuncType flag) { m_funcType = flag; }
     AstCFuncType funcType() const { return m_funcType; }
     bool	isInline() const { return m_isInline; }
diff --git a/src/V3CCtors.cpp b/src/V3CCtors.cpp
index c53e5ce40..3bb982af2 100644
--- a/src/V3CCtors.cpp
+++ b/src/V3CCtors.cpp
@@ -23,6 +23,8 @@
 //	for all AstCoverDecl, move the declaration into a _configure_coverage AstCFunc.
 //	For each variable that needs reset, add a AstCReset node.
 //
+//	For primary inputs, add _eval_debug_assertions.
+//
 //	This transformation honors outputSplitCFuncs.
 //*************************************************************************
 #include "config_build.h"
@@ -101,8 +103,46 @@ private:
 
 //######################################################################
 
+void V3CCtors::evalAsserts() {
+    AstNodeModule* modp = v3Global.rootp()->modulesp();  // Top module
+    AstCFunc* funcp = new AstCFunc(modp->fileline(), "_eval_debug_assertions", NULL, "void");
+    funcp->declPrivate(true);
+    funcp->isStatic(false);
+    funcp->slow(false);
+    funcp->ifdef("VL_DEBUG");
+    modp->addStmtp(funcp);
+    for (AstNode* np = modp->stmtsp(); np; np = np->nextp()) {
+	if (AstVar* varp = np->castVar()) {
+	    if (varp->isPrimaryIn() && !varp->isSc()) {
+		if (AstBasicDType* basicp = varp->dtypeSkipRefp()->castBasicDType()) {
+		    int storedWidth = basicp->widthAlignBytes() * 8;
+		    int lastWordWidth = varp->width() % storedWidth;
+		    if (lastWordWidth != 0) {
+			// if (signal & CONST(upper_non_clean_mask)) { fail; }
+			AstNode* newp = new AstVarRef(varp->fileline(), varp, false);
+			if (varp->isWide()) {
+			    newp = new AstWordSel(varp->fileline(), newp,
+						  new AstConst(varp->fileline(), varp->widthWords()-1));
+			}
+			uint64_t value = VL_MASK_Q(storedWidth) & ~VL_MASK_Q(lastWordWidth);
+			V3Number num (varp->fileline(), storedWidth, value);
+			newp = new AstAnd(varp->fileline(), newp,
+					  new AstConst(varp->fileline(), num));
+			AstNodeIf* ifp = new AstIf(varp->fileline(), newp,
+						   new AstCStmt(varp->fileline(), "Verilated::overWidthError(\""+varp->prettyName()+"\");"));
+			ifp->branchPred(AstBranchPred::BP_UNLIKELY);
+			newp = ifp;
+			funcp->addStmtsp(newp);
+		    }
+		}
+	    }
+	}
+    }
+}
+
 void V3CCtors::cctorsAll() {
     UINFO(2,__FUNCTION__<<": "<<endl);
+    evalAsserts();
     for (AstNodeModule* modp = v3Global.rootp()->modulesp(); modp; modp=modp->nextp()->castNodeModule()) {
         // Process each module in turn
         {
diff --git a/src/V3CCtors.h b/src/V3CCtors.h
index ab7764b11..c8c844f5b 100644
--- a/src/V3CCtors.h
+++ b/src/V3CCtors.h
@@ -30,6 +30,8 @@
 class V3CCtors {
 public:
     static void cctorsAll();
+private:
+    static void evalAsserts();
 };
 
 
diff --git a/src/V3EmitC.cpp b/src/V3EmitC.cpp
index 9b430f089..dc0a252ae 100644
--- a/src/V3EmitC.cpp
+++ b/src/V3EmitC.cpp
@@ -872,6 +872,7 @@ class EmitCImp : EmitCStmts {
 	splitSizeInc(nodep);
 
 	puts("\n");
+	if (nodep->ifdef()!="") puts("#ifdef "+nodep->ifdef()+"\n");
 	if (nodep->isInline()) puts("VL_INLINE_OPT ");
 	puts(nodep->rtnTypeVoid()); puts(" ");
 	puts(modClassName(m_modp)+"::"+nodep->name()
@@ -908,6 +909,7 @@ class EmitCImp : EmitCStmts {
 
 	//puts("__Vm_activity = true;\n");
 	puts("}\n");
+	if (nodep->ifdef()!="") puts("#endif // "+nodep->ifdef()+"\n");
     }
 
     void emitChangeDet() {
@@ -1731,14 +1733,18 @@ void EmitCImp::emitSensitives() {
 
 void EmitCImp::emitWrapEval(AstNodeModule* modp) {
     puts("\nvoid "+modClassName(modp)+"::eval() {\n");
+    puts("VL_DEBUG_IF(VL_DBG_MSGF(\"+++++TOP Evaluate "+modClassName(modp)+"::eval\\n\"); );\n");
     puts(EmitCBaseVisitor::symClassVar()+" = this->__VlSymsp;  // Setup global symbol table\n");
     puts(EmitCBaseVisitor::symTopAssign()+"\n");
+    puts("#ifdef VL_DEBUG\n");
+    putsDecoration("// Debug assertions\n");
+    puts("_eval_debug_assertions();\n");
+    puts("#endif // VL_DEBUG\n");
     putsDecoration("// Initialize\n");
     puts("if (VL_UNLIKELY(!vlSymsp->__Vm_didInit)) _eval_initial_loop(vlSymsp);\n");
     if (v3Global.opt.inhibitSim()) {
 	puts("if (VL_UNLIKELY(__Vm_inhibitSim)) return;\n");
     }
-    puts("VL_DEBUG_IF(VL_DBG_MSGF(\"+++++TOP Evaluate "+modClassName(modp)+"::eval\\n\"); );\n");
 
     if (v3Global.opt.threads()) {  // THREADED-TODO move to per-train
 	uint32_t trainId = 0;
@@ -1859,9 +1865,11 @@ void EmitCImp::emitIntFuncDecls(AstNodeModule* modp) {
 	AstCFunc* funcp = *it;
 	if (!funcp->dpiImport()) {  // DPI is prototyped in __Dpi.h
 	    ofp()->putsPrivate(funcp->declPrivate());
+	    if (funcp->ifdef()!="") puts("#ifdef "+funcp->ifdef()+"\n");
 	    if (funcp->isStatic()) puts("static ");
 	    puts(funcp->rtnTypeVoid()); puts(" ");
 	    puts(funcp->name()); puts("("+cFuncArgs(funcp)+");\n");
+	    if (funcp->ifdef()!="") puts("#endif // "+funcp->ifdef()+"\n");
 	}
     }
 }
diff --git a/test_regress/t/t_trace_cat.cpp b/test_regress/t/t_trace_cat.cpp
index 08786d9ea..27978f647 100644
--- a/test_regress/t/t_trace_cat.cpp
+++ b/test_regress/t/t_trace_cat.cpp
@@ -43,7 +43,7 @@ int main(int argc, char **argv, char **env) {
     top->clk = 0;
 
     while (main_time < 190) {  // Creates 2 files
-	top->clk   = ~top->clk;
+        top->clk = !top->clk;
 	top->eval();
 
 	if ((main_time % 100) == 0) {
diff --git a/test_regress/t/t_trace_timescale.cpp b/test_regress/t/t_trace_timescale.cpp
index 7c9b86a5f..284a8515a 100644
--- a/test_regress/t/t_trace_timescale.cpp
+++ b/test_regress/t/t_trace_timescale.cpp
@@ -32,7 +32,7 @@ int main(int argc, char **argv, char **env) {
     top->clk = 0;
 
     while (main_time < 190*VL_TIME_MULTIPLIER) {
-	top->clk   = ~top->clk;
+        top->clk = !top->clk;
 	top->eval();
 	tfp->dump((unsigned int)(main_time));
 	// Advance by 0.5 time units, to make sure our fractional
diff --git a/test_regress/t/t_var_overwidth_bad.cpp b/test_regress/t/t_var_overwidth_bad.cpp
new file mode 100644
index 000000000..e75242abd
--- /dev/null
+++ b/test_regress/t/t_var_overwidth_bad.cpp
@@ -0,0 +1,44 @@
+// -*- mode: C++; c-file-style: "cc-mode" -*-
+//*************************************************************************
+//
+// Copyright 2010-2011 by Wilson Snyder. This program is free software; you can
+// redistribute it and/or modify it under the terms of either the GNU
+// Lesser General Public License Version 3 or the Perl Artistic License.
+// Version 2.0.
+//
+// Verilator is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+//*************************************************************************
+
+#include "Vt_var_overwidth_bad.h"
+#include "verilated.h"
+
+//======================================================================
+
+double main_time;
+
+double sc_time_stamp () {
+    return main_time;
+}
+
+int main(int argc, char **argv, char **env) {
+    Verilated::debug(0);
+
+    VM_PREFIX* topp = new VM_PREFIX ("");  // Note null name - we're flattening it out
+
+    main_time = 0;
+
+    topp->clk = 0;
+    topp->eval();
+    main_time += 10;
+
+    topp->clk = 0x2; // ILLEGAL
+    topp->eval();
+    topp->final();
+
+    delete topp; topp=NULL;
+    exit(0L);
+}
diff --git a/test_regress/t/t_var_overwidth_bad.pl b/test_regress/t/t_var_overwidth_bad.pl
new file mode 100755
index 000000000..16a1b18c2
--- /dev/null
+++ b/test_regress/t/t_var_overwidth_bad.pl
@@ -0,0 +1,23 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2010 by Wilson Snyder. This program is free software; you can
+# redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+
+compile (
+    make_main => 0,
+    verilator_flags2 => ["--exe $Self->{t_dir}/t_var_overwidth_bad.cpp"],
+    );
+
+execute (
+    fails=>1,
+    expect=>
+qr{%Error: unknown:0: Testbench C set input 'clk' to value that overflows what the signal's width can fit
+Aborting....*}
+    );
+
+ok(1);
+1;
diff --git a/test_regress/t/t_var_overwidth_bad.v b/test_regress/t/t_var_overwidth_bad.v
new file mode 100644
index 000000000..52fe460f7
--- /dev/null
+++ b/test_regress/t/t_var_overwidth_bad.v
@@ -0,0 +1,19 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// Copyright 2010 by Wilson Snyder. This program is free software; you can
+// redistribute it and/or modify it under the terms of either the GNU
+// Lesser General Public License Version 3 or the Perl Artistic License
+// Version 2.0.
+
+module t (/*AUTOARG*/
+   // Inputs
+   clk
+   );
+
+   input clk;
+
+   always @ (posedge clk) begin
+      $write("*-* All Finished *-*\n");
+      $finish;
+   end
+endmodule