diff --git a/src/raster/r.fuzzy.set/fuzzy.c b/src/raster/r.fuzzy.set/fuzzy.c
index 958b8ae9ec..50c7f7e6a5 100644
--- a/src/raster/r.fuzzy.set/fuzzy.c
+++ b/src/raster/r.fuzzy.set/fuzzy.c
@@ -1,5 +1,6 @@
 #include <grass/glocale.h>
 #include "local_proto.h"
+#include <math.h>
 
 float fuzzy(FCELL cell)
 {
@@ -42,7 +43,7 @@ float fuzzy(FCELL cell)
     }
 
     if (type == SSHAPE) {
-        m = pow(2, exp(2 * abs(shape)));
+        m = pow(2, exp(2 * fabs(shape)));
         return (shape < 0) ? 1 - pow(cos(x * PI2), m) : pow(sin(x * PI2), m);
     }