diff --git a/modules/libpr0n/src/imgFrame.cpp b/modules/libpr0n/src/imgFrame.cpp
index 6677a3829844..995eb460528e 100644
--- a/modules/libpr0n/src/imgFrame.cpp
+++ b/modules/libpr0n/src/imgFrame.cpp
@@ -168,7 +168,7 @@ imgFrame::~imgFrame()
 }
 
 nsresult imgFrame::Init(PRInt32 aX, PRInt32 aY, PRInt32 aWidth, PRInt32 aHeight, 
-                        gfxASurface::gfxImageFormat aFormat, PRInt8 aPaletteDepth /* = 0 */)
+                        gfxASurface::gfxImageFormat aFormat, PRUint8 aPaletteDepth /* = 0 */)
 {
   // assert for properties that should be verified by decoders, warn for properties related to bad content
   if (!AllowedImageSize(aWidth, aHeight))
diff --git a/modules/libpr0n/src/imgFrame.h b/modules/libpr0n/src/imgFrame.h
index fb934e2055c7..799c84210f39 100644
--- a/modules/libpr0n/src/imgFrame.h
+++ b/modules/libpr0n/src/imgFrame.h
@@ -62,7 +62,7 @@ public:
   imgFrame();
   ~imgFrame();
 
-  nsresult Init(PRInt32 aX, PRInt32 aY, PRInt32 aWidth, PRInt32 aHeight, gfxASurface::gfxImageFormat aFormat, PRInt8 aPaletteDepth = 0);
+  nsresult Init(PRInt32 aX, PRInt32 aY, PRInt32 aWidth, PRInt32 aHeight, gfxASurface::gfxImageFormat aFormat, PRUint8 aPaletteDepth = 0);
   nsresult Optimize();
 
   void Draw(gfxContext *aContext, gfxPattern::GraphicsFilter aFilter,
@@ -188,7 +188,7 @@ private: // data
   PRInt32      mDisposalMethod;
 
   gfxASurface::gfxImageFormat mFormat;
-  PRInt8       mPaletteDepth;
+  PRUint8      mPaletteDepth;
   PRInt8       mBlendMethod;
   PRPackedBool mSinglePixel;
   PRPackedBool mNeverUseDeviceSurface;