diff --git a/src/gl/scene/gl_scene.cpp b/src/gl/scene/gl_scene.cpp
index 329ac6484..af03af86e 100644
--- a/src/gl/scene/gl_scene.cpp
+++ b/src/gl/scene/gl_scene.cpp
@@ -800,8 +800,8 @@ sector_t * GLSceneDrawer::RenderViewpoint (AActor * camera, GL_IRECT * bounds, f
 	GLRenderer->mAngles.Roll.Degrees = r_viewpoint.Angles.Roll.Degrees;
 
 	// Scroll the sky
-	GLRenderer->mSky1Pos = (double)fmod(screen->FrameTime * level.skyspeed1, 1024.f) * 90./256.;
-	GLRenderer->mSky2Pos = (double)fmod(screen->FrameTime * level.skyspeed2, 1024.f) * 90./256.;
+	GLRenderer->mSky1Pos = (double)fmod((double)screen->FrameTime * (double)level.skyspeed1, 1024.f) * 90./256.;
+	GLRenderer->mSky2Pos = (double)fmod((double)screen->FrameTime * (double)level.skyspeed2, 1024.f) * 90./256.;