diff --git a/source/lib/ogl.cpp b/source/lib/ogl.cpp
index fad2abb2fe..a42a56ba87 100755
--- a/source/lib/ogl.cpp
+++ b/source/lib/ogl.cpp
@@ -138,11 +138,14 @@ int ogl_get_gfx_info()
 	return 0;
 }
 
+
 const char* oglExtList()
 {
+	assert(exts && "call oglInit before using this function");
 	return exts;
 }
 
+
 // call after each video mode change
 void oglInit()
 {
diff --git a/source/lib/ogl.h b/source/lib/ogl.h
index 9d599de766..d7244d0cb9 100755
--- a/source/lib/ogl.h
+++ b/source/lib/ogl.h
@@ -113,7 +113,7 @@ extern int ogl_get_gfx_info();
 
 // return a NULL-terminated string (of unlimited length) containing
 // a space-separated list of supported extensions.
-const char* oglExtList();
+extern const char* oglExtList();
 
 #ifdef __cplusplus
 }