diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index 169330b9ec..1cc88502e5 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -288,6 +288,7 @@ pub mut:
 	return_type        table.Type
 	should_be_skipped  bool
 	generic_type       table.Type // TODO array, to support multiple types
+	free_receiver      bool // true if the receiver expression needs to be freed
 	// autofree_pregen    string
 	// autofree_vars      []AutofreeArgVar
 	// autofree_vars_ids  []int
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index ee9dbc85cf..ea6e80814c 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -952,7 +952,7 @@ pub fn (mut c Checker) call_expr(mut call_expr ast.CallExpr) table.Type {
 	*/
 	// Now call `call_method` or `call_fn` for specific checks.
 	typ := if call_expr.is_method { c.call_method(mut call_expr) } else { c.call_fn(mut call_expr) }
-	// autofree
+	// autofree: mark args that have to be freed (after saving them in tmp exprs)
 	free_tmp_arg_vars := c.pref.autofree && c.pref.experimental && !c.is_builtin_mod &&
 		call_expr.args.len > 0 && !call_expr.args[0].typ.has_flag(.optional)
 	if free_tmp_arg_vars {
@@ -965,6 +965,10 @@ pub fn (mut c Checker) call_expr(mut call_expr ast.CallExpr) table.Type {
 			}
 			call_expr.args[i].is_tmp_autofree = true
 		}
+		if call_expr.receiver_type == table.string_type && !(call_expr.left is ast.Ident ||
+			call_expr.left is ast.StringLiteral) {
+			call_expr.free_receiver = true
+		}
 	}
 	return typ
 }
diff --git a/vlib/v/gen/fn.v b/vlib/v/gen/fn.v
index 5c7215974c..5e7fec1444 100644
--- a/vlib/v/gen/fn.v
+++ b/vlib/v/gen/fn.v
@@ -443,7 +443,14 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 	} else if !node.receiver_type.is_ptr() && node.left_type.is_ptr() && node.name != 'str' {
 		g.write('/*rec*/*')
 	}
-	g.expr(node.left)
+	if node.free_receiver {
+		// The receiver expression needs to be freed, use the temp var.
+		fn_name := node.name.replace('.', '_')
+		arg_name := '_arg_expr_${fn_name}_0'
+		g.write('/*af receiver arg*/' + arg_name)
+	} else {
+		g.expr(node.left)
+	}
 	is_variadic := node.expected_arg_types.len > 0 && node.expected_arg_types[node.expected_arg_types.len -
 		1].has_flag(.variadic)
 	if node.args.len > 0 || is_variadic {
@@ -631,7 +638,15 @@ fn (mut g Gen) autofree_call_pregen(node ast.CallExpr) {
 	free_tmp_arg_vars = false // set the flag to true only if we have at least one arg to free
 	g.tmp_count2++
 	mut scope := g.file.scope.innermost(node.pos.pos)
-	for i, arg in node.args {
+	// prepend the receiver for now (TODO turn the receiver into a CallArg everywhere?)
+	mut args := [ast.CallArg{
+		typ: node.receiver_type
+		expr: node.left
+		is_tmp_autofree: node.free_receiver
+	}]
+	args << node.args
+	// for i, arg in node.args {
+	for i, arg in args {
 		if !arg.is_tmp_autofree {
 			continue
 		}
@@ -725,7 +740,7 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 					// Use these variables here.
 					fn_name := node.name.replace('.', '_')
 					// name := '_tt${g.tmp_count2}_arg_expr_${fn_name}_$i'
-					name := '_arg_expr_${fn_name}_$i'
+					name := '_arg_expr_${fn_name}_${i + 1}'
 					g.write('/*af arg*/' + name)
 				}
 			} else {
@@ -736,7 +751,7 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 				// TODO copypasta, move to an inline fn
 				fn_name := node.name.replace('.', '_')
 				// name := '_tt${g.tmp_count2}_arg_expr_${fn_name}_$i'
-				name := '_arg_expr_${fn_name}_$i'
+				name := '_arg_expr_${fn_name}_${i + 1}'
 				g.write('/*af arg2*/' + name)
 			} else {
 				g.expr(arg.expr)
diff --git a/vlib/v/tests/valgrind/1.strings_and_arrays.v b/vlib/v/tests/valgrind/1.strings_and_arrays.v
index 71bd76aa94..1cfb278f7e 100644
--- a/vlib/v/tests/valgrind/1.strings_and_arrays.v
+++ b/vlib/v/tests/valgrind/1.strings_and_arrays.v
@@ -3,7 +3,7 @@ fn return_array(array_arg []string) []int { // array argument must not be freed
 	return s
 }
 
-fn foo() {
+fn simple() {
 	nums := [1, 2, 3] // local array must be freed
 	println(nums)
 	nums_copy := nums // array assignments call .clone()
@@ -73,6 +73,15 @@ fn str_replace() {
 	*/
 }
 
+fn fooo(s string) {
+}
+
+fn str_replace2() {
+	mut s := 'hello world'
+	s = s.replace('hello', 'hi').replace('world', 'planet')
+	println(s)
+}
+
 fn reassign_str() {
 	mut s := 'a' + 'b'
 	s = 'x' + 'y' // 'a' + 'b' must be freed before the re-assignment
@@ -149,7 +158,7 @@ fn tt() {
 
 fn main() {
 	println('start')
-	foo()
+	simple()
 	str_tmp_expr()
 	str_tmp_expr_advanced()
 	str_tmp_expr_advanced_var_decl()
@@ -159,6 +168,7 @@ fn main() {
 	optional_str()
 	optional_return()
 	str_replace()
+	str_replace2()
 	if_cond()
 	addition_with_tmp_expr()
 	println('end')