diff --git a/include/gtkwave/fstapi.c b/include/gtkwave/fstapi.c
index 6b6adefc5..7bc7fd821 100644
--- a/include/gtkwave/fstapi.c
+++ b/include/gtkwave/fstapi.c
@@ -130,6 +130,7 @@ void **JenkinsIns(void *base_i, const unsigned char *mem, uint32_t length, uint3
 #define FST_HDR_TIMEZERO_SIZE           (8)
 #define FST_GZIO_LEN                    (32768)
 #define FST_HDR_FOURPACK_DUO_SIZE       (4*1024*1024)
+#define FST_ZWRAPPER_HDR_SIZE           (1+8+8)
 
 #if defined(__APPLE__) && defined(__MACH__)
 #define FST_MACOSX
@@ -4633,9 +4634,14 @@ if(sectype == FST_BL_ZWRAPPER)
                 }
 #endif
 
-        fstReaderFseeko(xc, xc->f, 1+8+8, SEEK_SET);
+        fstReaderFseeko(xc, xc->f, FST_ZWRAPPER_HDR_SIZE, SEEK_SET);
 #ifndef __MINGW32__
         fflush(xc->f);
+#else
+	/* Windows UCRT runtime library reads one byte ahead in the file
+	   even with buffering disabled and does not synchronise the
+	   file position after fseek. */
+	_lseek(fileno(xc->f), FST_ZWRAPPER_HDR_SIZE, SEEK_SET);
 #endif
 
         zfd = dup(fileno(xc->f));
@@ -6185,7 +6191,9 @@ struct fstReaderContext *xc = (struct fstReaderContext *)ctx;
 fst_off_t blkpos = 0, prev_blkpos;
 uint64_t beg_tim, end_tim, beg_tim2, end_tim2;
 int sectype;
+#ifdef FST_DEBUG
 unsigned int secnum = 0;
+#endif
 uint64_t seclen;
 uint64_t tsec_uclen = 0, tsec_clen = 0;
 uint64_t tsec_nitems;
@@ -6277,7 +6285,9 @@ for(;;)
                 }
 
         blkpos += seclen;
+#ifdef FST_DEBUG
         secnum++;
+#endif
         }
 
 xc->rvat_beg_tim = beg_tim;