diff --git a/dlls/msvcrt/math.c b/dlls/msvcrt/math.c
index bb450fad01c..efdde8e4afb 100644
--- a/dlls/msvcrt/math.c
+++ b/dlls/msvcrt/math.c
@@ -35,6 +35,7 @@
  * ====================================================
  */
 
+#include <assert.h>
 #include <complex.h>
 #include <stdio.h>
 #include <fenv.h>
@@ -5321,7 +5322,7 @@ static BOOL _setfp( unsigned int *cw, unsigned int cw_mask,
         if (*cw & _IC_AFFINE) newcw |= 0x1000;
     }
 
-    if (oldsw != newsw && newsw)
+    if (oldsw != newsw && (newsw & 0x3f))
     {
         struct {
             WORD control_word;
@@ -5338,6 +5339,8 @@ static BOOL _setfp( unsigned int *cw, unsigned int cw_mask,
             WORD unused5;
         } fenv;
 
+        assert(cw);
+
         __asm__ __volatile__( "fnstenv %0" : "=m" (fenv) );
         fenv.control_word = newcw;
         fenv.status_word = newsw;