diff --git a/gematria/granite/graph_builder.cc b/gematria/granite/graph_builder.cc
index 30f6c4ab..e4ead92c 100644
--- a/gematria/granite/graph_builder.cc
+++ b/gematria/granite/graph_builder.cc
@@ -485,9 +485,10 @@ void StrAppendList(std::stringstream& buffer, std::string_view list_name,
   buffer << list_name << " = [";
   bool first = true;
   for (const auto& item : items) {
-    if (!first) {
-      buffer << ",";
+    if (first) {
       first = false;
+    } else {
+      buffer << ", ";
     }
     buffer << item;
   }
diff --git a/gematria/granite/graph_builder_test.cc b/gematria/granite/graph_builder_test.cc
index 82cdbca4..60659de2 100644
--- a/gematria/granite/graph_builder_test.cc
+++ b/gematria/granite/graph_builder_test.cc
@@ -571,5 +571,37 @@ TEST_F(BasicBlockGraphBuilderTest, TwoNops) {
   )pb"))));
 }
 
+TEST_F(BasicBlockGraphBuilderTest, DebugString) {
+  CreateBuilder(OutOfVocabularyTokenBehavior::ReturnError());
+  ASSERT_TRUE(builder_->AddBasicBlock(BasicBlockFromProto(ParseTextProto(R"pb(
+    canonicalized_instructions: {
+      mnemonic: "LEA"
+      llvm_mnemonic: "LEA64r"
+      output_operands: { register_name: "RDI" }
+      input_operands: {
+        address: { base_register: "RBX" displacement: 8 scaling: 1 }
+      }
+    })pb"))));
+
+  const std::string expected_string =
+      "num_graphs = 1\n"
+      "num_nodes = 5\n"
+      "num_edges = 4\n"
+      "num_node_tokens = 16\n"
+      "num_nodes_per_block = [5]\n"
+      "num_edges_per_block = [4]\n"
+      "node_types = [NodeType::kInstruction, NodeType::kAddressOperand, "
+      "NodeType::kRegister, NodeType::kImmediate, NodeType::kRegister]\n"
+      "edge_senders = [2, 3, 1, 0]\n"
+      "edge_receivers = [1, 1, 0, 4]\n"
+      "edge_types = [EdgeType::kAddressBaseRegister, "
+      "EdgeType::kAddressDisplacement, EdgeType::kInputOperands, "
+      "EdgeType::kOutputOperands]\n"
+      "InstructionNodeMask = [1, 0, 0, 0, 0]\n"
+      "DeltaBlockIndex = [0]\n";
+
+  EXPECT_EQ(builder_->DebugString(), expected_string);
+}
+
 }  // namespace
 }  // namespace gematria