diff --git a/simgear/sound/sample_group.cxx b/simgear/sound/sample_group.cxx
index 70df9aa1..dbbb22ab 100644
--- a/simgear/sound/sample_group.cxx
+++ b/simgear/sound/sample_group.cxx
@@ -33,7 +33,7 @@
 #include "soundmgr_openal_private.hxx"
 #include "sample_group.hxx"
 
-#ifdef HAVE_STD_ISNAN
+#if defined(HAVE_STD_ISNAN) && !defined(HAVE_ISNAN)
 using std::isnan;
 #endif