diff --git a/simgear/compiler.h b/simgear/compiler.h
index 40bb0855..19b0c778 100644
--- a/simgear/compiler.h
+++ b/simgear/compiler.h
@@ -230,6 +230,7 @@
 
 #    define isnan _isnan
 #    define snprintf _snprintf
+#    define copysign _copysign
 
 #    pragma warning(disable: 4786) // identifier was truncated to '255' characters
 #    pragma warning(disable: 4244) // conversion from double to float