diff --git a/src/core.h b/src/core.h
index cc592461..29c47061 100644
--- a/src/core.h
+++ b/src/core.h
@@ -45,6 +45,21 @@
 #   define FP_USE(x) (void)(x)
 #endif
 
+/* Ensure isnan() is present even on systems that don't define it, or
+ * when -ffast-math is being used. */
+#if defined __FAST_MATH__
+#   undef isnan
+#endif
+#if !defined isnan
+#   define isnan isnan
+#   include <stdint.h>
+static inline int isnan(float f)
+{
+    union { float f; uint32_t x; } u = { f };
+    return (u.x << 1) > 0xff000000u;
+}
+#endif
+
 // Base types
 #include "trig.h"
 #include "half.h"
diff --git a/test/half.cpp b/test/half.cpp
index 8740857a..cf88ad36 100644
--- a/test/half.cpp
+++ b/test/half.cpp
@@ -17,19 +17,6 @@
 #include "core.h"
 #include "lol/unit.h"
 
-/* Ensure isnan() is present even on systems that don't define it, or
- * when -ffast-math is being used. */
-#if defined __FAST_MATH__
-#   undef isnan
-#endif
-#if !defined isnan
-static inline int isnan(float f)
-{
-    union { float f; uint32_t x; } u = { f };
-    return (u.x << 1) > 0xff000000u;
-}
-#endif
-
 namespace lol
 {