diff --git a/src/io/io.go b/src/io/io.go
index adc0c0d550..87ebe8c147 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -30,6 +30,9 @@ var ErrShortWrite = errors.New("short write")
 // ErrShortBuffer means that a read required a longer buffer than was provided.
 var ErrShortBuffer = errors.New("short buffer")
 
+// ErrBadWriteCount means that a write returned an impossible count.
+var ErrBadWriteCount = errors.New("Write returned impossible count")
+
 // EOF is the error returned by Read when no more input is available.
 // (Read must return EOF itself, not an error wrapping EOF,
 // because callers will test for EOF using ==.)
@@ -411,9 +414,13 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 		nr, er := src.Read(buf)
 		if nr > 0 {
 			nw, ew := dst.Write(buf[0:nr])
-			if nw > 0 {
-				written += int64(nw)
+			if nw < 0 || nr < nw {
+				nw = 0
+				if ew == nil {
+					ew = ErrBadWriteCount
+				}
 			}
+			written += int64(nw)
 			if ew != nil {
 				err = ew
 				break
diff --git a/src/io/io_test.go b/src/io/io_test.go
index 170513dcc0..a8399bcac6 100644
--- a/src/io/io_test.go
+++ b/src/io/io_test.go
@@ -429,3 +429,31 @@ func TestSectionReader_Size(t *testing.T) {
 		}
 	}
 }
+
+// largeWriter returns an invalid count that is larger than the number
+// of bytes provided (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)
+	}
+
+	want = errors.New("largeWriterError")
+	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)
+	}
+}