diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index e6e1287993b..ebafb4b5dc9 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -660,7 +660,7 @@ func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) refle
 	case *parse.PipeNode:
 		return s.validateType(s.evalPipeline(dot, arg), typ)
 	case *parse.IdentifierNode:
-		return s.evalFunction(dot, arg, arg, nil, zero)
+		return s.validateType(s.evalFunction(dot, arg, arg, nil, zero), typ)
 	case *parse.ChainNode:
 		return s.validateType(s.evalChainNode(dot, arg, nil, zero), typ)
 	}
diff --git a/src/text/template/exec_test.go b/src/text/template/exec_test.go
index abce27ff3dc..0f1ad62380a 100644
--- a/src/text/template/exec_test.go
+++ b/src/text/template/exec_test.go
@@ -531,6 +531,8 @@ var execTests = []execTest{
 	{"bug14a", "{{(nil).True}}", "", tVal, false},
 	{"bug14b", "{{$x := nil}}{{$x.anything}}", "", tVal, false},
 	{"bug14c", `{{$x := (1.0)}}{{$y := ("hello")}}{{$x.anything}}{{$y.true}}`, "", tVal, false},
+	// Didn't call validateType on function results. Issue 10800.
+	{"bug15", "{{valueString returnInt}}", "", tVal, false},
 }
 
 func zeroArgs() string {
@@ -570,6 +572,11 @@ func valueString(v string) string {
 	return "value is ignored"
 }
 
+// returnInt returns an int
+func returnInt() int {
+	return 7
+}
+
 func add(args ...int) int {
 	sum := 0
 	for _, x := range args {
@@ -611,6 +618,7 @@ func testExecute(execTests []execTest, template *Template, t *testing.T) {
 		"makemap":     makemap,
 		"mapOfThree":  mapOfThree,
 		"oneArg":      oneArg,
+		"returnInt":   returnInt,
 		"stringer":    stringer,
 		"typeOf":      typeOf,
 		"valueString": valueString,