diff --git a/gfx/src/thebes/nsThebesDeviceContext.cpp b/gfx/src/thebes/nsThebesDeviceContext.cpp
index 6b64093205aa..1f5433d958b4 100644
--- a/gfx/src/thebes/nsThebesDeviceContext.cpp
+++ b/gfx/src/thebes/nsThebesDeviceContext.cpp
@@ -659,6 +659,9 @@ nsThebesDeviceContext::SetDPI()
                     dpi = lDPI;
                 break;
 #endif
+            default:
+                NS_NOTREACHED("Unexpected printing surface type");
+                break;
         }
         dotsArePixels = PR_FALSE;
     } else {