diff --git a/src/sound/mididevices/music_opl_mididevice.cpp b/src/sound/mididevices/music_opl_mididevice.cpp
index f588618b1..a740548b3 100644
--- a/src/sound/mididevices/music_opl_mididevice.cpp
+++ b/src/sound/mididevices/music_opl_mididevice.cpp
@@ -44,6 +44,7 @@
 // MACROS ------------------------------------------------------------------
 
 #if defined(_DEBUG) && defined(_WIN32) && defined(_MSC_VER)
+void I_DebugPrint(const char *cp);
 #define DEBUGOUT(m,c,s,t) \
 	{ char foo[128]; mysnprintf(foo, countof(foo), m, c, s, t); I_DebugPrint(foo); }
 #else