diff --git a/src/mir/builder/calls/lowering.rs b/src/mir/builder/calls/lowering.rs
index 067bfb20..c0f4cdd7 100644
--- a/src/mir/builder/calls/lowering.rs
+++ b/src/mir/builder/calls/lowering.rs
@@ -94,9 +94,24 @@ impl MirBuilder {
     fn setup_function_params(&mut self, params: &[String]) {
         self.function_param_names.clear();
         if let Some(ref mut f) = self.current_function {
-            for p in params {
-                let pid = f.next_value_id();
-                f.params.push(pid);
+            // 📦 Hotfix 5: Use pre-populated params from MirFunction::new()
+            // Static methods have implicit receiver at params[0], so actual parameters start at offset
+            let receiver_offset = if f.params.is_empty() { 0 } else {
+                // If params already populated (by Hotfix 4+5), use them
+                if f.params.len() > params.len() { 1 } else { 0 }
+            };
+
+            for (idx, p) in params.iter().enumerate() {
+                let param_idx = receiver_offset + idx;
+                let pid = if param_idx < f.params.len() {
+                    // Use pre-allocated ValueId from MirFunction::new()
+                    f.params[param_idx]
+                } else {
+                    // Allocate new ValueId (fallback for non-static methods)
+                    let new_pid = f.next_value_id();
+                    f.params.push(new_pid);
+                    new_pid
+                };
                 self.variable_map.insert(p.clone(), pid);
                 self.function_param_names.insert(p.clone());
             }
diff --git a/src/mir/function.rs b/src/mir/function.rs
index d49160f6..03d10713 100644
--- a/src/mir/function.rs
+++ b/src/mir/function.rs
@@ -104,9 +104,17 @@ impl MirFunction {
         let total_value_ids = param_count + receiver_count;
         let initial_counter = total_value_ids.max(1);  // At least 1 to reserve ValueId(0) as sentinel
 
+        // 🔥 Hotfix 5: Pre-populate params vector with reserved ValueIds
+        // Without this, setup_function_params() will allocate NEW ValueIds starting from
+        // the already-incremented counter, causing signature/body mismatch.
+        let mut pre_params = Vec::new();
+        for i in 0..total_value_ids {
+            pre_params.push(ValueId::new(i));
+        }
+
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-            eprintln!("[MirFunction::new] fn='{}' params={}, receiver={}, total={}, initial_counter={}",
-                     signature.name, param_count, receiver_count, total_value_ids, initial_counter);
+            eprintln!("[MirFunction::new] fn='{}' params={}, receiver={}, total={}, initial_counter={}, pre_params={:?}",
+                     signature.name, param_count, receiver_count, total_value_ids, initial_counter, pre_params);
         }
 
         Self {
@@ -114,7 +122,7 @@ impl MirFunction {
             blocks,
             entry_block,
             locals: Vec::new(),
-            params: Vec::new(),
+            params: pre_params,  // ✅ Pre-populate instead of empty Vec
             next_value_id: initial_counter,
             metadata: FunctionMetadata::default(),
         }