diff --git a/internal/compiler/funchandler.go b/internal/compiler/funchandler.go
index 6d0b218..377d72e 100644
--- a/internal/compiler/funchandler.go
+++ b/internal/compiler/funchandler.go
@@ -386,42 +386,60 @@ func (fh *FunctionHandler) processStringArg(arg string, param *FuncParam, funcNa
 	return nil
 }
 
-// processVarArg handles variable arguments
 func (fh *FunctionHandler) processVarArg(sym *Symbol, param *FuncParam, funcName string, line preproc.Line, inAssigns, outAssigns *[]string) error {
-	// Check type compatibility
-	if (sym.IsByte() && param.Symbol.IsWord()) || (sym.IsWord() && param.Symbol.IsByte()) {
-		return fmt.Errorf("%s:%d: CALL %s: type mismatch for parameter %s", line.Filename, line.LineNo, funcName, param.Symbol.Name)
-	}
-
 	if sym.IsConst() {
 		return fmt.Errorf("%s:%d: CALL %s: cannot pass constant to function", line.Filename, line.LineNo, funcName)
 	}
 
-	// Generate IN assignments
+	// Generate IN assignments (sym -> param)
 	if param.Direction.Has(DirIn) {
 		*inAssigns = append(*inAssigns,
 			fmt.Sprintf("  lda %s", sym.FullName()),
 			fmt.Sprintf("  sta %s", param.Symbol.FullName()),
 		)
-		if sym.IsWord() {
-			*inAssigns = append(*inAssigns,
-				fmt.Sprintf("  lda %s+1", sym.FullName()),
-				fmt.Sprintf("  sta %s+1", param.Symbol.FullName()),
-			)
+		if param.Symbol.IsWord() {
+			if sym.IsWord() {
+				*inAssigns = append(*inAssigns,
+					fmt.Sprintf("  lda %s+1", sym.FullName()),
+					fmt.Sprintf("  sta %s+1", param.Symbol.FullName()),
+				)
+			} else {
+				// byte -> word: zero extend
+				*inAssigns = append(*inAssigns,
+					"  lda #0",
+					fmt.Sprintf("  sta %s+1", param.Symbol.FullName()),
+				)
+			}
+		} else if sym.IsWord() {
+			// word -> byte: lossy
+			fmt.Printf("%s:%d: warning: CALL %s: truncating word '%s' to byte parameter '%s'\n",
+				line.Filename, line.LineNo, funcName, sym.Name, param.Symbol.Name)
 		}
 	}
 
-	// Generate OUT assignments
+	// Generate OUT assignments (param -> sym)
 	if param.Direction.Has(DirOut) {
 		*outAssigns = append(*outAssigns,
 			fmt.Sprintf("  lda %s", param.Symbol.FullName()),
 			fmt.Sprintf("  sta %s", sym.FullName()),
 		)
 		if sym.IsWord() {
-			*outAssigns = append(*outAssigns,
-				fmt.Sprintf("  lda %s+1", param.Symbol.FullName()),
-				fmt.Sprintf("  sta %s+1", sym.FullName()),
-			)
+			if param.Symbol.IsWord() {
+				*outAssigns = append(*outAssigns,
+					fmt.Sprintf("  lda %s+1", param.Symbol.FullName()),
+					fmt.Sprintf("  sta %s+1", sym.FullName()),
+				)
+			} else {
+				// byte -> word: zero extend
+				*outAssigns = append(*outAssigns,
+					"  lda #0",
+					fmt.Sprintf("  sta %s+1", sym.FullName()),
+				)
+			}
+		} else if param.Symbol.IsWord() {
+			// word -> byte: lossy
+			fmt.Printf("%s:%d: warning: CALL %s: truncating word parameter '%s' to byte '%s'\n",
+				line.Filename, line.LineNo, funcName, param.Symbol.Name, sym.Name)
 		}
 	}
 
diff --git a/internal/compiler/funchandler_test.go b/internal/compiler/funchandler_test.go
index ad27d9c..0b78536 100644
--- a/internal/compiler/funchandler_test.go
+++ b/internal/compiler/funchandler_test.go
@@ -587,6 +587,8 @@ func TestHandleFuncCall_Errors(t *testing.T) {
 			line:    "CALL test ( 1 2 )",
 			wantErr: "expected 1 arguments, got 2",
 		},
+		//
+		/* This is no error anymore, just a warning.
 		{
 			name: "type mismatch",
 			setup: func(fh *FunctionHandler, st *SymbolTable) {
@@ -596,7 +598,7 @@ func TestHandleFuncCall_Errors(t *testing.T) {
 			},
 			line:    "CALL test ( wvar )",
 			wantErr: "type mismatch",
-		},
+		}, */
 		{
 			name: "const to out param",
 			setup: func(fh *FunctionHandler, st *SymbolTable) {