diff --git a/src/io/io.go b/src/io/io.go
index 7a5d7d1a3e..a6544ac7ce 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -413,8 +413,10 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 		if nr > 0 {
 			nw, ew := dst.Write(buf[0:nr])
 			if nw < 0 || nr < nw {
-				err = ErrBadWriteCount
-				break
+				nw = 0
+				if ew == nil {
+					ew = ErrBadWriteCount
+				}
 			}
 			written += int64(nw)
 			if ew != nil {
diff --git a/src/io/io_test.go b/src/io/io_test.go
index 170513dcc0..f3476610ba 100644
--- a/src/io/io_test.go
+++ b/src/io/io_test.go
@@ -429,3 +429,33 @@ func TestSectionReader_Size(t *testing.T) {
 		}
 	}
 }
+
+// largeWriter returns an invalid count that is larger than the number
+// of bytes requested (issue 39978).
+type largeWriter struct {
+	err error
+}
+
+func (w largeWriter) Write(p []byte) (int, error) {
+	return len(p) + 1, w.err
+}
+
+func TestCopyLargeWriter(t *testing.T) {
+	want := ErrBadWriteCount
+	rb := new(Buffer)
+	wb := largeWriter{}
+	rb.WriteString("hello, world.")
+	if _, err := Copy(wb, rb); err != want {
+		t.Errorf("Copy Error: got %v, want %v", err, want)
+	}
+}
+
+func TestCopyLargeWriterError(t *testing.T) {
+	want := errors.New("largerWriteError")
+	rb := new(Buffer)
+	wb := largeWriter{err: want}
+	rb.WriteString("hello, world.")
+	if _, err := Copy(wb, rb); err != want {
+		t.Errorf("Copy Error: got %v, want %v", err, want)
+	}
+}