diff --git a/src/compton.c b/src/compton.c
index 7ac5daa..f086eb5 100644
--- a/src/compton.c
+++ b/src/compton.c
@@ -4016,7 +4016,7 @@ get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass) {
     ps->o.wintype_shadow[WINTYPE_DND] = false;
   if (fading_enable)
     wintype_arr_enable(ps->o.wintype_fade);
-  if (!isnan(cfgtmp.menu_opacity)) {
+  if (!safe_isnan(cfgtmp.menu_opacity)) {
     ps->o.wintype_opacity[WINTYPE_DROPDOWN_MENU] = cfgtmp.menu_opacity;
     ps->o.wintype_opacity[WINTYPE_POPUP_MENU] = cfgtmp.menu_opacity;
   }
diff --git a/src/utils.h b/src/utils.h
index 5f033f0..fbb88b2 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -4,6 +4,17 @@
 #include <ctype.h>
 #include <stddef.h>
 #include <stdlib.h>
+#include <math.h>
+
+#ifdef __FAST_MATH__
+#warning Use of -ffast-math can cause rendering error or artifacts, \
+  therefore it is not recommended.
+#endif
+
+__attribute__((optimize("-fno-fast-math")))
+static inline bool safe_isnan(double a) {
+  return isnan(a);
+}
 
 /**
  * Normalize an int value to a specific range.
diff --git a/src/win.c b/src/win.c
index a1e3d09..b778b28 100644
--- a/src/win.c
+++ b/src/win.c
@@ -324,7 +324,7 @@ void win_calc_opacity(session_t *ps, win *w) {
     // Try obeying opacity property and window type opacity firstly
     if (w->has_opacity_prop)
       opacity = w->opacity_prop;
-    else if (!isnan(ps->o.wintype_opacity[w->window_type]))
+    else if (!safe_isnan(ps->o.wintype_opacity[w->window_type]))
       opacity = ps->o.wintype_opacity[w->window_type] * OPAQUE;
     else {
       // Respect active_opacity only when the window is physically focused