diff --git a/src/manifest/mod.rs b/src/manifest/mod.rs
index bf715a81..383f3bb7 100644
--- a/src/manifest/mod.rs
+++ b/src/manifest/mod.rs
@@ -628,10 +628,13 @@ impl CrateData {
         // Check if a `package.json` was already generated by wasm-bindgen, if so
         // we merge the NPM dependencies already specified in it.
         let existing_deps = if pkg_file_path.exists() {
-            // It's just a map of dependency names to versions
-            Some(serde_json::from_str::<HashMap<String, String>>(
-                &fs::read_to_string(&pkg_file_path)?,
-            )?)
+            #[derive(Deserialize)]
+            struct HasDependencies {
+                dependencies: HashMap<String, String>,
+            }
+            let package =
+                serde_json::from_str::<HasDependencies>(&fs::read_to_string(&pkg_file_path)?)?;
+            Some(package.dependencies)
         } else {
             None
         };