diff --git a/utils/funcexp/functor_str.h b/utils/funcexp/functor_str.h
index ab6f3631e..e3c97cca8 100644
--- a/utils/funcexp/functor_str.h
+++ b/utils/funcexp/functor_str.h
@@ -108,7 +108,7 @@ protected:
 		}
         exponent = (int)floor(log10( fabs(floatVal)));
         base = floatVal * pow(10, -1.0*exponent);
-        if (isnan(exponent) || isnan(base))
+        if (std::isnan(exponent) || std::isnan(base))
         {
             snprintf(buf, 20, "%f", floatVal);
             fFloatStr = execplan::removeTrailing0(buf, 20);