diff --git a/compiler/gen_wasm/src/backend.rs b/compiler/gen_wasm/src/backend.rs
index e7e39531e4..c1b34873a8 100644
--- a/compiler/gen_wasm/src/backend.rs
+++ b/compiler/gen_wasm/src/backend.rs
@@ -40,7 +40,8 @@ pub struct WasmBackend<'a> {
     module: WasmModule<'a>,
     layout_ids: LayoutIds<'a>,
     next_constant_addr: u32,
-    preloaded_fn_index_map: MutMap<&'a str, u32>,
+    fn_index_offset: u32,
+    preloaded_functions_map: MutMap<&'a [u8], u32>,
     proc_symbols: Vec<'a, (Symbol, u32)>,
     helper_proc_gen: CodeGenHelp<'a>,
 
@@ -62,6 +63,8 @@ impl<'a> WasmBackend<'a> {
         layout_ids: LayoutIds<'a>,
         proc_symbols: Vec<'a, (Symbol, u32)>,
         module: WasmModule<'a>,
+        fn_index_offset: u32,
+        preloaded_functions_map: MutMap<&'a [u8], u32>,
         helper_proc_gen: CodeGenHelp<'a>,
     ) -> Self {
         WasmBackend {
@@ -73,7 +76,8 @@ impl<'a> WasmBackend<'a> {
 
             layout_ids,
             next_constant_addr: CONST_SEGMENT_BASE_ADDR,
-            preloaded_fn_index_map: MutMap::default(),
+            fn_index_offset,
+            preloaded_functions_map,
             proc_symbols,
             helper_proc_gen,
 
@@ -542,8 +546,7 @@ impl<'a> WasmBackend<'a> {
                     for (roc_proc_index, (ir_sym, linker_sym_index)) in
                         self.proc_symbols.iter().enumerate()
                     {
-                        let wasm_fn_index =
-                            self.module.code.preloaded_count + roc_proc_index as u32;
+                        let wasm_fn_index = self.fn_index_offset + roc_proc_index as u32;
                         if ir_sym == func_sym {
                             let num_wasm_args = param_types.len();
                             let has_return_val = ret_type.is_some();
@@ -1461,7 +1464,7 @@ impl<'a> WasmBackend<'a> {
     ) {
         let num_wasm_args = param_types.len();
         let has_return_val = ret_type.is_some();
-        let fn_index = self.preloaded_fn_index_map[name];
+        let fn_index = self.preloaded_functions_map[name.as_bytes()];
         let linker_symbol_index = u32::MAX;
 
         self.code_builder
diff --git a/compiler/gen_wasm/src/lib.rs b/compiler/gen_wasm/src/lib.rs
index b718ff9f43..6996b2a13b 100644
--- a/compiler/gen_wasm/src/lib.rs
+++ b/compiler/gen_wasm/src/lib.rs
@@ -92,8 +92,17 @@ pub fn build_module_help<'a>(
         fn_index += 1;
     }
 
+    // Pre-load the WasmModule with data from the platform & builtins object file
     let initial_module = WasmModule::preload(env.arena, preload_bytes);
-    let main_function_index = maybe_main_fn_index.unwrap() + initial_module.code.preloaded_count;
+
+    // Adjust Wasm function indices to account for functions from the object file
+    let runtime_import_fn_count: u32 = initial_module.import.function_count(); // to be imported at runtime (e.g. WASI)
+    let fn_index_offset: u32 = runtime_import_fn_count + initial_module.code.preloaded_count;
+
+    // Get a map of name to index for the preloaded functions
+    // Assumes the preloaded object file has all symbols exported, as per `zig build-lib -dymamic`
+    let preloaded_functions_map: MutMap<&'a [u8], u32> =
+        initial_module.export.function_index_map(env.arena);
 
     let mut backend = WasmBackend::new(
         env,
@@ -101,6 +110,8 @@ pub fn build_module_help<'a>(
         layout_ids,
         proc_symbols,
         initial_module,
+        fn_index_offset,
+        preloaded_functions_map,
         CodeGenHelp::new(env.arena, IntWidth::I32, env.module_id),
     );
 
@@ -137,6 +148,7 @@ pub fn build_module_help<'a>(
 
     let module = backend.into_module();
 
+    let main_function_index = maybe_main_fn_index.unwrap() + fn_index_offset;
     Ok((module, main_function_index))
 }