diff --git a/src/fn_native.rs b/src/fn_native.rs
index 4d2cf19b..820e5dc7 100644
--- a/src/fn_native.rs
+++ b/src/fn_native.rs
@@ -261,7 +261,9 @@ impl CallableFunction {
     pub fn is_pure(&self) -> bool {
         match self {
             Self::Pure(_) => true,
-            Self::Method(_) | Self::Iterator(_) | Self::Plugin(_) => false,
+            Self::Method(_) | Self::Iterator(_) => false,
+
+            Self::Plugin(p) => !p.is_method_call(),
 
             #[cfg(not(feature = "no_function"))]
             Self::Script(_) => false,
@@ -271,7 +273,9 @@ impl CallableFunction {
     pub fn is_method(&self) -> bool {
         match self {
             Self::Method(_) => true,
-            Self::Pure(_) | Self::Iterator(_) | Self::Plugin(_) => false,
+            Self::Pure(_) | Self::Iterator(_) => false,
+
+            Self::Plugin(p) => p.is_method_call(),
 
             #[cfg(not(feature = "no_function"))]
             Self::Script(_) => false,
diff --git a/src/packages/string_more.rs b/src/packages/string_more.rs
index 087e8e90..99af280d 100644
--- a/src/packages/string_more.rs
+++ b/src/packages/string_more.rs
@@ -22,12 +22,12 @@ macro_rules! gen_concat_functions {
                 use super::super::*;
 
                 #[export_fn]
-                pub fn append_func(x: ImmutableString, y: $arg_type) -> String {
+                pub fn append_func(x: &mut ImmutableString, y: $arg_type) -> String {
                     super::super::add_append(x, y)
                 }
 
                 #[export_fn]
-                pub fn prepend_func(x: $arg_type, y: ImmutableString) -> String {
+                pub fn prepend_func(x: &mut $arg_type, y: ImmutableString) -> String {
                     super::super::add_prepend(x, y)
                 }
             }
@@ -120,10 +120,10 @@ def_package!(crate:MoreStringPackage:"Additional string utilities, including str
     );
 });
 
-fn add_prepend<T: Display>(x: T, y: ImmutableString) -> String {
+fn add_prepend<T: Display>(x: &mut T, y: ImmutableString) -> String {
     format!("{}{}", x, y)
 }
-fn add_append<T: Display>(x: ImmutableString, y: T) -> String {
+fn add_append<T: Display>(x: &mut ImmutableString, y: T) -> String {
     format!("{}{}", x, y)
 }
 
diff --git a/tests/plugins.rs b/tests/plugins.rs
index 3379b411..5331287b 100644
--- a/tests/plugins.rs
+++ b/tests/plugins.rs
@@ -7,6 +7,10 @@ use rhai::{Engine, EvalAltResult, INT};
 mod special_array_package {
     use rhai::{Array, INT};
 
+    #[rhai_fn(get = "foo", return_raw)]
+    pub fn foo(array: &mut Array) -> Result<Dynamic, Box<EvalAltResult>> {
+        Ok(array[0].clone())
+    }
     #[rhai_fn(name = "test")]
     pub fn len(array: &mut Array, mul: INT) -> INT {
         (array.len() as INT) * mul
@@ -53,6 +57,7 @@ fn test_plugins_package() -> Result<(), Box<EvalAltResult>> {
 
     reg_functions!(engine += greet::single(INT, bool, char));
 
+    assert_eq!(engine.eval::<INT>("let a = [1, 2, 3]; a.foo")?, 1);
     assert_eq!(engine.eval::<INT>("let a = [1, 2, 3]; test(a, 2)")?, 6);
     assert_eq!(engine.eval::<INT>("2 + 2")?, 5);
     assert_eq!(