diff --git a/gDP.cpp b/gDP.cpp
index bef43ddb..6f9e24bb 100644
--- a/gDP.cpp
+++ b/gDP.cpp
@@ -60,7 +60,7 @@ void gDPSetOtherMode( u32 mode0, u32 mode1 )
 
 void gDPSetPrimDepth( u16 z, u16 dz )
 {
-	gDP.primDepth.z = min(1.0f, max(0.0f, (_FIXED2FLOAT(_SHIFTR(z, 0, 15), 15) - gSP.viewport.vtrans[2]) / gSP.viewport.vscale[2]));
+	gDP.primDepth.z = min(1.0f, max(-1.0f, (_FIXED2FLOAT(_SHIFTR(z, 0, 15), 15) - gSP.viewport.vtrans[2]) / gSP.viewport.vscale[2]));
 	gDP.primDepth.deltaZ = dz;
 
 #ifdef DEBUG