diff --git a/src/ast.c b/src/ast.c
index 8e84986..a0ba01c 100644
--- a/src/ast.c
+++ b/src/ast.c
@@ -78,13 +78,25 @@ void ast_num_print(ASTNumData* data, int i) {
     INDENT_END;
 }
 
-ASTExcData ast_exc_data_init(char* msg) { return (ASTExcData)msg; }
+ASTExcData* ast_exc_data_init(char* msg, AST* trace) {
+    ASTExcData* data = malloc(sizeof(ASTExcData));
+    data->msg = msg;
+    data->trace = trace;
+    return data;
+}
 
-void ast_exc_print(ASTExcData data, int i) {
+void ast_exc_print(ASTExcData* data, int i) {
     INDENT_BEGIN(i);
 
     INDENT_TITLE("ASTExcData", data);
-    INDENT_FIELD("msg", "\"%s\"", data);
+    INDENT_FIELD("msg", "\"%s\"", data->msg);
+    if (data->trace == NULL) {
+        INDENT_FIELD("trace", "%p", NULL)
+    } else {
+        INDENT_FIELD_EXT_NONL_START("trace");
+        ast_print_i(data->trace, i + 1);
+        INDENT_FIELD_NONL_END;
+    }
     INDENT_END;
 }
 
diff --git a/src/builtin.c b/src/builtin.c
index c243602..2ad2358 100644
--- a/src/builtin.c
+++ b/src/builtin.c
@@ -6,15 +6,19 @@
 #include <stdio.h>
 
 AST* builtin_sum(size_t argc, AST** argv) {
-    log_dbg("Got here");
     ASTNumData total = 0;
 
     for (int i = 0; i < argc; i++) {
         AST* arg = exec_exp(argv[i]);
+        if (arg->type == AST_TYPE_EXC)
+            return ast_init(
+                AST_TYPE_EXC,
+                ast_exc_data_init("`sum` encountered an exception.", arg)
+            );
         if (arg->type != AST_TYPE_NUM)
             return ast_init(
                 AST_TYPE_EXC,
-                ast_exc_data_init("Sum can't sum some non-num arguments.")
+                ast_exc_data_init("Sum can't sum some non-num arguments.", NULL)
             );
 
         total += *(ASTNumData*)arg->data;
@@ -26,19 +30,30 @@ AST* builtin_sum(size_t argc, AST** argv) {
 AST* builtin_sub(size_t argc, AST** argv) {
     log_dbg("Got here");
     AST* first = exec_exp(*argv);
+    if (first->type == AST_TYPE_EXC)
+        return ast_init(
+            AST_TYPE_EXC,
+            ast_exc_data_init("`sub` encountered an exception.", first)
+        );
     if (first->type != AST_TYPE_NUM)
         return ast_init(
-            AST_TYPE_EXC, ast_exc_data_init("Can't subtract non-num arguments.")
+            AST_TYPE_EXC,
+            ast_exc_data_init("Can't subtract non-num arguments.", NULL)
         );
 
     ASTNumData total = *(ASTNumData*)first->data;
 
     for (int i = 1; i < argc; i++) {
         AST* arg = exec_exp(argv[i]);
+        if (arg->type == AST_TYPE_EXC)
+            return ast_init(
+                AST_TYPE_EXC,
+                ast_exc_data_init("`sub` encountered an exception.", arg)
+            );
         if (arg->type != AST_TYPE_NUM)
             return ast_init(
                 AST_TYPE_EXC,
-                ast_exc_data_init("Can't subtract non-num arguments.")
+                ast_exc_data_init("Can't subtract non-num arguments.", NULL)
             );
 
         total -= *(ASTNumData*)arg->data;
@@ -50,19 +65,30 @@ AST* builtin_sub(size_t argc, AST** argv) {
 AST* builtin_mul(size_t argc, AST** argv) {
     log_dbg("Got here");
     AST* first = exec_exp(*argv);
+    if (first->type == AST_TYPE_EXC)
+        return ast_init(
+            AST_TYPE_EXC,
+            ast_exc_data_init("`mul` encountered an expection.", first)
+        );
     if (first->type != AST_TYPE_NUM)
         return ast_init(
-            AST_TYPE_EXC, ast_exc_data_init("Can't multiply non-num arguments.")
+            AST_TYPE_EXC,
+            ast_exc_data_init("Can't multiply non-num arguments.", NULL)
         );
 
     ASTNumData total = *(ASTNumData*)first->data;
 
     for (int i = 1; i < argc; i++) {
         AST* arg = exec_exp(argv[i]);
+        if (arg->type == AST_TYPE_EXC)
+            return ast_init(
+                AST_TYPE_EXC,
+                ast_exc_data_init("`mul` encountered an execption.", arg)
+            );
         if (arg->type != AST_TYPE_NUM)
             return ast_init(
                 AST_TYPE_EXC,
-                ast_exc_data_init("Can't multiply non-num arguments.")
+                ast_exc_data_init("Can't multiply non-num arguments.", NULL)
             );
 
         total *= *(ASTNumData*)arg->data;
@@ -74,19 +100,30 @@ AST* builtin_mul(size_t argc, AST** argv) {
 AST* builtin_div(size_t argc, AST** argv) {
     log_dbg("Got here");
     AST* first = exec_exp(*argv);
+    if (first->type == AST_TYPE_EXC)
+        return ast_init(
+            AST_TYPE_EXC,
+            ast_exc_data_init("`div` encountered an exception.", first)
+        );
     if (first->type != AST_TYPE_NUM)
         return ast_init(
-            AST_TYPE_EXC, ast_exc_data_init("Can't divide non-num arguments.")
+            AST_TYPE_EXC,
+            ast_exc_data_init("Can't divide non-num arguments.", NULL)
         );
 
     ASTNumData total = *(ASTNumData*)first->data;
 
     for (int i = 1; i < argc; i++) {
         AST* arg = exec_exp(argv[i]);
+        if (arg->type == AST_TYPE_EXC)
+            return ast_init(
+                AST_TYPE_EXC,
+                ast_exc_data_init("`div` encountered an exception.", arg)
+            );
         if (arg->type != AST_TYPE_NUM)
             return ast_init(
                 AST_TYPE_EXC,
-                ast_exc_data_init("Can't divide non-num arguments.")
+                ast_exc_data_init("Can't divide non-num arguments.", NULL)
             );
 
         total /= *(ASTNumData*)arg->data;
diff --git a/src/exec.c b/src/exec.c
index 1dfc890..50ef05c 100644
--- a/src/exec.c
+++ b/src/exec.c
@@ -69,14 +69,15 @@ AST* exec_call(AST* ast) {
 
     if (fdef == NULL)
         return ast_init(
-            AST_TYPE_EXC, ast_exc_data_init(strdup("No such function found."))
+            AST_TYPE_EXC, ast_exc_data_init("No such function found.", NULL)
         );
 
     switch (fdef->type) {
         case AST_TYPE_BIF:
             ASTBIFData bifdata = fdef->data;
             return bifdata(argc, argv);
-        default: return ast_init(AST_TYPE_EXC, ast_exc_data_init("Good job!"));
+        default:
+            return ast_init(AST_TYPE_EXC, ast_exc_data_init("Good job!", NULL));
     }
 }
 
@@ -113,7 +114,7 @@ AST* exec_vref(AST* ast) {
             msg, sizeof(msg), "Could not find value in scope for `%s`.",
             vref->to
         );
-        return ast_init(AST_TYPE_EXC, msg);
+        return ast_init(AST_TYPE_EXC, ast_exc_data_init(msg, NULL));
     }
 
     return exec_exp(found);
diff --git a/src/include/ast.h b/src/include/ast.h
index 55e7343..1e33ee2 100644
--- a/src/include/ast.h
+++ b/src/include/ast.h
@@ -51,13 +51,16 @@ void ast_num_data_destroy(ASTNumData* num);
 void ast_num_print(ASTNumData*, int i);
 
 // An exception.
-typedef char* ASTExcData;
+typedef struct ASTEXCDATA {
+    char* msg; // The exception message.
+    AST* trace; // The previous exception.
+} ASTExcData;
 // Create a new `ASTExecData.
-ASTExcData ast_exc_data_init(char* msg);
+ASTExcData* ast_exc_data_init(char* msg, AST* trace);
 // Destroy an `ASTExecData`.
 void ast_exc_data_destroy(ASTExcData* exc);
 // Print an `ASTExecData`.
-void ast_exc_print(ASTExcData, int i);
+void ast_exc_print(ASTExcData*, int i);
 
 // A built-in function.
 typedef AST* (*ASTBIFData)(size_t argc, AST** argv);
diff --git a/src/stack.c b/src/stack.c
index 92290e6..8102e82 100644
--- a/src/stack.c
+++ b/src/stack.c
@@ -21,7 +21,6 @@ void stack_destroy(Stack* stack) {
 }
 
 void stack_push(Stack* stack, void* val) {
-    log_dbgf("pushed to stack, inc ln to %ld", stack->ln);
     if (stack->ln >= STACK_MAX) {
         log_dbgf("Ran out of stack (max: %d)", STACK_MAX);
         return;
@@ -29,6 +28,7 @@ void stack_push(Stack* stack, void* val) {
 
     stack->buf[stack->ln] = val;
     stack->ln++;
+    log_dbgf("pushed to stack, inc ln to %ld", stack->ln);
 }
 
 void* stack_pop(Stack* stack) {