diff --git a/src/ast.c b/src/ast.c
index da03a9a..f8b72fb 100644
--- a/src/ast.c
+++ b/src/ast.c
@@ -27,6 +27,13 @@ AST* ast_init(ASTType type, void* data) {
     ast->data = data;
     ast->scope = NULL;
 
+    if (ast->type > AST_TYPE_MAX) {
+        log_dbgf(
+            "Attempted to create invalid AST (%i > %i) to GC: ast:%p",
+            ast->type, AST_TYPE_MAX, ast
+        );
+    }
+
     return ast;
 }
 
@@ -56,10 +63,7 @@ void ast_destroy(AST* ast) {
     }
 
     // If there're no more `AST`s linked to the scope, free.
-    if (ast->scope) {
-        ast->scope->uses--;
-        if (!ast->scope->uses) { scope_destroy_psv(ast->scope); }
-    }
+    if (ast->scope && !--ast->scope->uses) scope_destroy_psv(ast->scope);
 
     free(ast);
 }
@@ -76,6 +80,7 @@ void ast_destroy_psv(AST* ast) {
         case AST_TYPE_FDEF:  ast_fdef_data_destroy_psv(ast->data); break;
         case AST_TYPE_ARG:   ast_arg_data_destroy(ast->data); break;
         case AST_TYPE_BIF:   ast_bif_data_destroy(ast->data); break;
+        case AST_TYPE_EXC:   ast_exc_data_destroy(ast->data); break;
         default:
             log_dbgf("Unknown ast type %d (max: %d)", ast->type, AST_TYPE_MAX);
     }
@@ -127,13 +132,18 @@ void ast_num_print(ASTNumData* data, int i) {
     INDENT_END;
 }
 
-ASTExcData* ast_exc_data_init(char* msg, AST* trace) {
+ASTExcData* ast_exc_data_init(const char* msg, AST* trace) {
     ASTExcData* data = malloc(sizeof(ASTExcData));
     data->msg = msg;
     data->trace = trace;
     return data;
 }
 
+void ast_exc_data_destroy(ASTExcData* exc) {
+    // `msg` is static, and `trace` will get freed in GC.
+    free(exc);
+}
+
 void ast_exc_print(ASTExcData* data, int i) {
     INDENT_BEGIN(i);
 
@@ -177,8 +187,8 @@ void ast_call_data_destroy(ASTCallData* call) {
 
 void ast_call_data_destroy_psv(ASTCallData* call) {
     if (!call) return;
-    free(call->to);
-    free(call->argv);
+    call->to = NULL;
+    call->argv = NULL;
     free(call);
 }
 
diff --git a/src/gc.c b/src/gc.c
index 4331486..6624e9b 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -10,6 +10,8 @@ GC* gclist = NULL;
 
 void gc_destroy(GC* gc) { free(gc); }
 
+void f() {}
+
 void* gc_alloc(size_t sz, GCType type) {
     void* mem = malloc(sz);
     GC* gc = malloc(sizeof(GC));
@@ -28,7 +30,17 @@ void gc_hack_free() {
         GC* gc = gclist;
         gclist = gclist->nxt;
         switch (gc->type) {
-            case GC_TYPE_AST:   ast_destroy_psv(gc->p); break;
+            case GC_TYPE_AST:
+                f();
+                if (((AST*)gc->p)->type > AST_TYPE_MAX) {
+                    log_dbgf(
+                        "Attempted to free invalid AST (%i > %i) to GC: gc:%p "
+                        "ast:%p",
+                        ((AST*)gc->p)->type, AST_TYPE_MAX, gc, gc->p
+                    );
+                }
+                ast_destroy_psv(gc->p);
+                break;
             case GC_TYPE_SCOPE: scope_destroy_psv(gc->p); break;
         }
         gc_destroy(gc);
diff --git a/src/grammar.y b/src/grammar.y
index 9854f38..1019d5a 100644
--- a/src/grammar.y
+++ b/src/grammar.y
@@ -124,7 +124,13 @@ block:
 exp:
     NUM { $$ = ast_init(AST_TYPE_NUM, ast_num_data_init($1)); }
 
-    //| BLOCKS exp BLOCKE { $$ = $2; }
+    // Function definitions.
+    | WORD GROUPS arg GROUPE EQ exp {
+        size_t argc = $3->ln;
+        AST** argv = $3->buf;
+        argarr_destroypsv($3);
+        $$ = ast_init(AST_TYPE_FDEF, ast_fdef_data_init($1, argc, argv, $6));
+    }
 
     | BLOCKS block BLOCKE {
         $$ = ast_init(AST_TYPE_BLOCK, ast_block_data_init((AST**) $2->buf, $2->ln));
@@ -193,11 +199,4 @@ exp:
         strcpy(to, "div");
         $$ = ast_init(AST_TYPE_CALL, ast_call_data_init(to, 2, argv));
     }
-
-    | WORD GROUPS arg GROUPE EQ exp {
-        size_t argc = $3->ln;
-        AST** argv = $3->buf;
-        argarr_destroypsv($3);
-        $$ = ast_init(AST_TYPE_FDEF, ast_fdef_data_init($1, argc, argv, $6));
-    }
 %%
diff --git a/src/include/ast.h b/src/include/ast.h
index eaff6cc..b71ec39 100644
--- a/src/include/ast.h
+++ b/src/include/ast.h
@@ -25,7 +25,7 @@ typedef enum {
     AST_TYPE_BLOCK, // A block of code (scope).
     AST_TYPE_FDEF,  // A function definition.
     AST_TYPE_ARG,   // A definition argument.
-    AST_TYPE_MAX = AST_TYPE_FDEF,
+    AST_TYPE_MAX = AST_TYPE_ARG,
 } ASTType;
 
 // An Abstract Syntax Tree.
@@ -60,11 +60,11 @@ void ast_num_print(ASTNumData*, int i);
 
 // An exception.
 typedef struct ASTEXCDATA {
-    char* msg;  // The exception message.
+    const char* msg;  // The exception message.
     AST* trace; // The previous exception.
 } ASTExcData;
-// Create a new `ASTExecData.
-ASTExcData* ast_exc_data_init(char* msg, AST* trace);
+// Create a new `ASTExecData. `msg` should be static.
+ASTExcData* ast_exc_data_init(const char* msg, AST* trace);
 // Destroy an `ASTExecData`.
 void ast_exc_data_destroy(ASTExcData* exc);
 // Print an `ASTExecData`.