diff --git a/docs/wasm-shaper.md b/docs/wasm-shaper.md
index 07705a454..ad72a1f90 100644
--- a/docs/wasm-shaper.md
+++ b/docs/wasm-shaper.md
@@ -35,12 +35,12 @@ harfbuzz-wasm = { path = "your-harfbuzz-source/src/wasm/rust/harfbuzz-wasm"}
 use wasm_bindgen::prelude::*;
 
 #[wasm_bindgen]
-pub fn shape(_font_ref: u32, _buf_ref: u32, _features: u32, _num_features: u32) -> i32 {
+pub fn shape(_shape_plan: u32, _font_ref: u32, _buf_ref: u32, _features: u32) -> i32 {
     1 // success!
 }
 ```
 
-This exports a shaping function which takes two arguments, tokens representing the font and the buffer, and returns a status value. We can pass these tokens back to Harfbuzz in order to use its native functions on the font and buffer objects. More on native functions later - let's get this shaper compiled and added into a font:
+This exports a shaping function which takes four arguments, tokens representing the shaping plan, the font and the buffer, and returns a status value. We can pass these tokens back to Harfbuzz in order to use its native functions on the font and buffer objects. More on native functions later - let's get this shaper compiled and added into a font:
 
 * To compile the shaper, run `wasm-pack build --target nodejs`:
 
@@ -85,7 +85,7 @@ In debugging builds of Harfbuzz, we can print some output from the web assembly
 use harfbuzz_wasm::debug;
 
 #[wasm_bindgen]
-pub fn shape(_font_ref: u32, _buf_ref: u32, _features: u32, _num_features: u32) -> i32 {
+pub fn shape(shape_plan, font_ref: u32, buf_ref: u32, _features: u32) -> i32 {
     debug("Hello from Rust!\n");
     1
 }
@@ -106,7 +106,7 @@ use wasm_bindgen::prelude::*;
 use harfbuzz_wasm::{Font, GlyphBuffer};
 
 #[wasm_bindgen]
-pub fn shape(_font_ref: u32, _buf_ref: u32, _features: u32, _num_features: u32) -> i32 {
+pub fn shape(_shape_plan:u32, font_ref: u32, buf_ref: u32, _features: u32) -> i32 {
     let font = Font::from_ref(font_ref);
     let mut buffer = GlyphBuffer::from_ref(buf_ref);
     for mut item in buffer.glyphs.iter_mut() {
diff --git a/src/wasm/rust/harfbuzz-wasm/src/lib.rs b/src/wasm/rust/harfbuzz-wasm/src/lib.rs
index 4b34e1d2d..3fe1fa545 100644
--- a/src/wasm/rust/harfbuzz-wasm/src/lib.rs
+++ b/src/wasm/rust/harfbuzz-wasm/src/lib.rs
@@ -42,7 +42,7 @@ extern "C" {
     fn font_get_glyph_h_advance(font: u32, glyph: u32) -> i32;
     fn font_get_glyph_v_advance(font: u32, glyph: u32) -> i32;
     fn face_copy_table(font: u32, tag: u32) -> Blob;
-    fn buffer_copy_contents(buffer: u32) -> CBufferContents;
+    fn buffer_copy_contents(buffer: u32, cbuffer: *mut CBufferContents) -> bool;
     fn buffer_set_contents(buffer: u32, cbuffer: &CBufferContents) -> bool;
     fn debugprint(s: *const u8);
     fn shape_with(
@@ -203,7 +203,15 @@ impl<T: BufferItem> Buffer<T> {
     /// The `Buffer` struct implements Drop, meaning that when the shaping
     /// function is finished, the buffer contents are sent back to Harfbuzz.
     pub fn from_ref(ptr: u32) -> Self {
-        let c_contents: CBufferContents = unsafe { buffer_copy_contents(ptr) };
+        let mut c_contents = CBufferContents {
+            info: std::ptr::null_mut(),
+            position: std::ptr::null_mut(),
+            length: 0,
+        };
+
+        unsafe {
+            buffer_copy_contents(ptr, &mut c_contents) || panic!("Couldn't copy buffer contents")
+        };
         let positions: Vec<CGlyphPosition> = unsafe {
             std::slice::from_raw_parts(c_contents.position, c_contents.length as usize).to_vec()
         };