diff --git a/intrange.go b/intrange.go
index 9173bdf..dc7d33a 100644
--- a/intrange.go
+++ b/intrange.go
@@ -265,6 +265,15 @@ func checkForStmt(pass *analysis.Pass, forStmt *ast.ForStmt) {
 		replacement = fmt.Sprintf("range %s", rangeX)
 	}
 
+	if isFunctionOrMethodCall(operand) {
+		pass.Report(analysis.Diagnostic{
+			Pos:     forStmt.Pos(),
+			Message: msg + "\nBecause the key is returned by a function or method, take care to consider side effects.",
+		})
+
+		return
+	}
+
 	pass.Report(analysis.Diagnostic{
 		Pos:     forStmt.Pos(),
 		Message: msg,
@@ -315,12 +324,11 @@ func checkRangeStmt(pass *analysis.Pass, rangeStmt *ast.RangeStmt) {
 		return
 	}
 
-	fn, ok := x.Fun.(*ast.Ident)
-	if !ok {
+	if _, ok = x.Fun.(*ast.Ident); !ok {
 		return
 	}
 
-	if fn.Name != "len" || len(x.Args) != 1 {
+	if !isLen(x) {
 		return
 	}
 
@@ -385,7 +393,7 @@ func checkRangeStmt(pass *analysis.Pass, rangeStmt *ast.RangeStmt) {
 func findNExpr(expr ast.Expr) ast.Expr {
 	switch e := expr.(type) {
 	case *ast.CallExpr:
-		if fun, ok := e.Fun.(*ast.Ident); ok && fun.Name == "len" && len(e.Args) == 1 {
+		if isLen(e) {
 			return findNExpr(e.Args[0])
 		}
 
@@ -528,19 +536,7 @@ func isNumberLit(exp ast.Expr) bool {
 	case *ast.CallExpr:
 		switch fun := lit.Fun.(type) {
 		case *ast.Ident:
-			switch fun.Name {
-			case
-				"int",
-				"int8",
-				"int16",
-				"int32",
-				"int64",
-				"uint",
-				"uint8",
-				"uint16",
-				"uint32",
-				"uint64":
-			default:
+			if !isIntCast(fun) {
 				return false
 			}
 		default:
@@ -575,19 +571,7 @@ func compareNumberLit(exp ast.Expr, val int) bool {
 	case *ast.CallExpr:
 		switch fun := lit.Fun.(type) {
 		case *ast.Ident:
-			switch fun.Name {
-			case
-				"int",
-				"int8",
-				"int16",
-				"int32",
-				"int64",
-				"uint",
-				"uint8",
-				"uint16",
-				"uint32",
-				"uint64":
-			default:
+			if !isIntCast(fun) {
 				return false
 			}
 		default:
@@ -627,3 +611,49 @@ func operandToString(
 
 	return t.String() + "(" + s + ")"
 }
+
+func isFunctionOrMethodCall(expr ast.Expr) bool {
+	e, ok := expr.(*ast.CallExpr)
+	if !ok {
+		return false
+	}
+
+	fun, ok := e.Fun.(*ast.Ident)
+	if !ok {
+		return true
+	}
+
+	if isLen(e) || isIntCast(fun) {
+		return false
+	}
+
+	return true
+}
+
+func isIntCast(ident *ast.Ident) bool {
+	switch ident.Name {
+	case
+		"int",
+		"int8",
+		"int16",
+		"int32",
+		"int64",
+		"uint",
+		"uint8",
+		"uint16",
+		"uint32",
+		"uint64":
+		return true
+	default:
+		return false
+	}
+}
+
+func isLen(exp *ast.CallExpr) bool {
+	fun, ok := exp.Fun.(*ast.Ident)
+	if !ok {
+		return false
+	}
+
+	return fun.Name == "len" && len(exp.Args) == 1
+}
diff --git a/testdata/main.go b/testdata/main.go
index b7c2077..7b0d21b 100644
--- a/testdata/main.go
+++ b/testdata/main.go
@@ -664,6 +664,19 @@ func issue33() {
 	}
 }
 
+func issue48() {
+	foo := struct {
+		Len func() int
+	}{
+		Len: func() int {
+			return 10
+		},
+	}
+
+	for i := 0; i < foo.Len(); i++ { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)\nBecause the key is returned by a function or method, take care to consider side effects.`
+	}
+}
+
 func issue50() {
 	x := 10
 	i := &x
diff --git a/testdata/main.go.golden b/testdata/main.go.golden
index 6e03687..ed4e1a9 100644
--- a/testdata/main.go.golden
+++ b/testdata/main.go.golden
@@ -58,11 +58,11 @@ func main() {
 	for range 10 { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
 	}
 
-	for i := range calculate(10) { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
+	for i := int32(0); i < calculate(10); i++ { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
 		fmt.Println(i)
 	}
 
-	for range calculate(10) { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
+	for i := int32(0); i < calculate(10); i++ { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
 	}
 
 	for i := int32(0); i <= calculate(9); i++ {
@@ -321,11 +321,11 @@ func main() {
 	for range 1*x { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
 	}
 
-	for i := range calculate(1*x) { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
+	for i := int32(0); i < calculate(1*x); i++  { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
 		fmt.Println(i)
 	}
 
-	for range calculate(1*x) { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
+	for i := int32(0); i < calculate(1*x); i++ { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
 	}
 
 	for i := range uint32(x) { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
@@ -532,7 +532,7 @@ func main() {
 
 // https://github.com/ckaznocha/intrange/issues/16
 func issue16(service protoreflect.ServiceDescriptor) {
-	for i := range service.Methods().Len() { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
+	for i := 0; i < service.Methods().Len(); i++ { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)`
 		print(i)
 	}
 }
@@ -664,6 +664,19 @@ func issue33() {
 	}
 }
 
+func issue48() {
+	foo := struct {
+		Len func() int
+	}{
+		Len: func() int {
+			return 10
+		},
+	}
+
+	for i := 0; i < foo.Len(); i++ { // want `for loop can be changed to use an integer range \(Go 1\.22\+\)\nBecause the key is returned by a function or method, take care to consider side effects.`
+	}
+}
+
 func issue50() {
 	x := 10
 	i := &x