diff --git a/skimage/exposure/unwrap_2d_ljmu.c b/skimage/exposure/unwrap_2d_ljmu.c
index 6be7966a..0b633268 100644
--- a/skimage/exposure/unwrap_2d_ljmu.c
+++ b/skimage/exposure/unwrap_2d_ljmu.c
@@ -19,6 +19,10 @@
 #include <string.h>
 #include <math.h>
 
+#ifndef M_PI
+#define M_PI 3.1415926535897932384626433832795
+#endif
+
 #define PI M_PI
 #define TWOPI (2 * M_PI)