diff --git a/lintcmd/cache/prog.go b/lintcmd/cache/prog.go
index 3c134cbb..a080abfd 100644
--- a/lintcmd/cache/prog.go
+++ b/lintcmd/cache/prog.go
@@ -226,7 +226,7 @@ func (c *ProgCache) writeToChild(req *cacheprog.Request, resc chan<- *cacheprog.
 			return err
 		}
 		if err := e.Close(); err != nil {
-			return nil
+			return err
 		}
 		if wrote != req.BodySize {
 			return fmt.Errorf("short write writing body to GOCACHEPROG for action %x, output %x: wrote %v; expected %v",
diff --git a/lintcmd/cache/prog_test.go b/lintcmd/cache/prog_test.go
new file mode 100644
index 00000000..0f150f3e
--- /dev/null
+++ b/lintcmd/cache/prog_test.go
@@ -0,0 +1,54 @@
+package cache
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"errors"
+	"strings"
+	"testing"
+
+	"honnef.co/go/tools/lintcmd/cache/cacheprog"
+)
+
+var errTestWriter = errors.New("test writer failed")
+
+type failingWriter struct{}
+
+func (failingWriter) Write([]byte) (int, error) { return 0, errTestWriter }
+
+func TestWriteToChildReturnsBase64CloseError(t *testing.T) {
+	const groups = 1000
+	bodySize := groups*3 + 1 // The base64 encoder buffers a final padded group.
+	body := strings.Repeat("x", bodySize)
+	req := &cacheprog.Request{
+		ID:       1,
+		Command:  cacheprog.CmdPut,
+		ActionID: []byte{1},
+		OutputID: []byte{2},
+		BodySize: int64(bodySize),
+	}
+	var header bytes.Buffer
+	if err := json.NewEncoder(&header).Encode(req); err != nil {
+		t.Fatal(err)
+	}
+	// Leave three bytes available after the request header and the unpadded
+	// base64 body, so the final padded group makes the buffer flush and fail
+	// from Encoder.Close.
+	bufferSize := header.Len() + 2 + groups*4 + 3
+	pc := &ProgCache{
+		inFlight: map[int64]chan<- *cacheprog.Response{},
+		bw:       bufio.NewWriterSize(failingWriter{}, bufferSize),
+	}
+	pc.jenc = json.NewEncoder(pc.bw)
+	got := pc.writeToChild(&cacheprog.Request{
+		Command:  cacheprog.CmdPut,
+		ActionID: []byte{1},
+		OutputID: []byte{2},
+		Body:     strings.NewReader(body),
+		BodySize: int64(bodySize),
+	}, make(chan *cacheprog.Response, 1))
+	if !errors.Is(got, errTestWriter) {
+		t.Fatalf("writeToChild error = %v, want %v", got, errTestWriter)
+	}
+}