diff --git a/gfx/thebes/gfxPattern.cpp b/gfx/thebes/gfxPattern.cpp
index f81da84f952..9aaa6d880a5 100644
--- a/gfx/thebes/gfxPattern.cpp
+++ b/gfx/thebes/gfxPattern.cpp
@@ -111,7 +111,7 @@ gfxPattern::GetInverseMatrix() const
 }
 
 Pattern*
-gfxPattern::GetPattern(DrawTarget *aTarget,
+gfxPattern::GetPattern(const DrawTarget *aTarget,
                        Matrix *aOriginalUserToDevice)
 {
   Matrix patternToUser = mPatternToUserSpace;
diff --git a/gfx/thebes/gfxPattern.h b/gfx/thebes/gfxPattern.h
index 037131d39d3..ee2c9759049 100644
--- a/gfx/thebes/gfxPattern.h
+++ b/gfx/thebes/gfxPattern.h
@@ -52,7 +52,7 @@ public:
      * was set. When this is nullptr it is assumed the transform is identical
      * to the current transform.
      */
-    mozilla::gfx::Pattern *GetPattern(mozilla::gfx::DrawTarget *aTarget,
+    mozilla::gfx::Pattern *GetPattern(const mozilla::gfx::DrawTarget *aTarget,
                                       mozilla::gfx::Matrix *aOriginalUserToDevice = nullptr);
     bool IsOpaque();