diff --git a/src/KSPACE/fft3d.cpp b/src/KSPACE/fft3d.cpp
index 638dd46f8e..dd5cb9a9ff 100644
--- a/src/KSPACE/fft3d.cpp
+++ b/src/KSPACE/fft3d.cpp
@@ -840,7 +840,7 @@ void bifactor(int n, int *factor1, int *factor2)
 {
   int n1,n2,facmax;
 
-  facmax = static_cast<int> (sqrt(n));
+  facmax = static_cast<int> (sqrt((double) n));
 
   for (n1 = facmax; n1 > 0; n1--) {
     n2 = n/n1;