add required conversions; bug in compiler let it slip through

R=r
DELTA=2  (0 added, 0 deleted, 2 changed)
OCL=34860
CL=34864

diff --git a/src/pkg/image/png/reader.go b/src/pkg/image/png/reader.go
index d715f0c4a2158cc9c29d13e316ed977b305c79b2..4be7e130c98ce5f04c5f3bbd6c9977056584d3c3 100644 (file)
--- a/src/pkg/image/png/reader.go
+++ b/src/pkg/image/png/reader.go
@@ -62,7 +62,7 @@ type decoder struct {
 type FormatError string
 
 func (e FormatError) String() string {
-       return "invalid PNG format: " + e;
+       return "invalid PNG format: " + string(e);
 }
 
 var chunkOrderError = FormatError("chunk out of order")
@@ -80,7 +80,7 @@ func (e IDATDecodingError) String() string {
 type UnsupportedError string
 
 func (e UnsupportedError) String() string {
-       return "unsupported PNG feature: " + e;
+       return "unsupported PNG feature: " + string(e);
 }
 
 // Big-endian.