diff --git a/src/net/textproto/reader.go b/src/net/textproto/reader.go
index ac47f007007..65974f9cc21 100644
--- a/src/net/textproto/reader.go
+++ b/src/net/textproto/reader.go
@@ -28,7 +28,6 @@ type Reader struct {
 // should be reading from an io.LimitReader or similar Reader to bound
 // the size of responses.
 func NewReader(r *bufio.Reader) *Reader {
-	commonHeaderOnce.Do(initCommonHeader)
 	return &Reader{R: r}
 }
 
@@ -579,8 +578,6 @@ func (r *Reader) upcomingHeaderNewlines() (n int) {
 // If s contains a space or invalid header field bytes, it is
 // returned without modifications.
 func CanonicalMIMEHeaderKey(s string) string {
-	commonHeaderOnce.Do(initCommonHeader)
-
 	// Quick check for canonical encoding.
 	upper := true
 	for i := 0; i < len(s); i++ {
@@ -642,6 +639,7 @@ func canonicalMIMEHeaderKey(a []byte) string {
 		a[i] = c
 		upper = c == '-' // for next time
 	}
+	commonHeaderOnce.Do(initCommonHeader)
 	// The compiler recognizes m[string(byteSlice)] as a special
 	// case, so a copy of a's bytes into a new string does not
 	// happen in this map lookup:
diff --git a/src/net/textproto/reader_test.go b/src/net/textproto/reader_test.go
index 3124d438fa5..d11d40f1cf1 100644
--- a/src/net/textproto/reader_test.go
+++ b/src/net/textproto/reader_test.go
@@ -8,8 +8,10 @@ import (
 	"bufio"
 	"bytes"
 	"io"
+	"net"
 	"reflect"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -324,6 +326,33 @@ func TestCommonHeaders(t *testing.T) {
 	}
 }
 
+func TestIssue46363(t *testing.T) {
+	// Regression test for data race reported in issue 46363:
+	// ReadMIMEHeader reads commonHeader before commonHeader has been initialized.
+	// Run this test with the race detector enabled to catch the reported data race.
+
+	// Reset commonHeaderOnce, so that commonHeader will have to be initialized
+	commonHeaderOnce = sync.Once{}
+	commonHeader = nil
+
+	// Test for data race by calling ReadMIMEHeader and CanonicalMIMEHeaderKey concurrently
+
+	// Send MIME header over net.Conn
+	r, w := net.Pipe()
+	go func() {
+		// ReadMIMEHeader calls canonicalMIMEHeaderKey, which reads from commonHeader
+		NewConn(r).ReadMIMEHeader()
+	}()
+	w.Write([]byte("A: 1\r\nB: 2\r\nC: 3\r\n\r\n"))
+
+	// CanonicalMIMEHeaderKey calls commonHeaderOnce.Do(initCommonHeader) which initializes commonHeader
+	CanonicalMIMEHeaderKey("a")
+
+	if commonHeader == nil {
+		t.Fatal("CanonicalMIMEHeaderKey should initialize commonHeader")
+	}
+}
+
 var clientHeaders = strings.Replace(`Host: golang.org
 Connection: keep-alive
 Cache-Control: max-age=0