diff --git a/skimage/exposure/unwrap_3d_ljmu.c b/skimage/exposure/unwrap_3d_ljmu.c
index 1874c051..52934eed 100644
--- a/skimage/exposure/unwrap_3d_ljmu.c
+++ b/skimage/exposure/unwrap_3d_ljmu.c
@@ -21,6 +21,10 @@
 #include <string.h>
 #include <math.h>
 
+#ifndef M_PI
+#define M_PI 3.1415926535897932384626433832795
+#endif
+
 #define PI M_PI
 #define TWOPI (2 * M_PI)