diff --git a/src/lib/smartsheet.ts b/src/lib/smartsheet.ts
index 705fdec..37d1f2a 100644
--- a/src/lib/smartsheet.ts
+++ b/src/lib/smartsheet.ts
@@ -137,39 +137,58 @@ async function request<T>(
  *   - A bare ID: "123456789012345"
  *   - A URL:    "https://app.smartsheet.com/sheets/abc123?..."
  *
- * Smartsheet share URLs include an opaque slug (e.g. "abc123"). We
- * can't resolve that slug without an API call (the sheet meta call
- * will fail with 404 anyway), so we just take the last path segment
- * and pass it as the sheet ID. If the user pasted a slug URL, the
- * "Test Connection" UI will surface the 404 and prompt for a numeric
- * ID instead. (This matches what most Smartsheet integrations do.)
+ * Smartsheet has TWO kinds of sheet URLs:
+ *   - `https://app.smartsheet.com/sheets/<numeric-id>` — the real ID.
+ *     Works against the REST API.
+ *   - `https://app.smartsheet.com/sheets/<share-slug>` — a 30+ char
+ *     opaque slug for sharing with non-users. The REST API does NOT
+ *     accept these; calling `/2.0/sheets/<slug>` returns a 2xx with
+ *     an empty/malformed body, which used to crash our parser with
+ *     "Cannot read properties of undefined (reading 'id')". We detect
+ *     that case here and surface a clear, actionable error so the
+ *     admin pastes the numeric ID instead.
+ *
+ * The numeric ID can be found in the sheet's URL once the user is
+ * signed in to Smartsheet, or via File → Properties in the Smartsheet
+ * UI.
  */
 export function extractSheetId(input: string): string {
   const trimmed = input.trim();
   if (!trimmed) {
     throw new Error("Sheet ID or URL is required");
   }
-  // Plain numeric ID
+  // Plain numeric ID — short-circuit.
   if (/^\d+$/.test(trimmed)) return trimmed;
 
-  // URL form
+  // URL form — pull the segment after "sheets", or fall back to the
+  // last path segment.
+  let candidate = trimmed;
   try {
     const url = new URL(trimmed);
     const segments = url.pathname.split("/").filter(Boolean);
-    // Find the segment after "sheets"
     const sheetsIdx = segments.findIndex((s) => s === "sheets");
     if (sheetsIdx >= 0 && segments[sheetsIdx + 1]) {
-      return segments[sheetsIdx + 1];
-    }
-    // Last segment as a fallback
-    if (segments.length > 0) {
-      return segments[segments.length - 1];
+      candidate = segments[sheetsIdx + 1];
+    } else if (segments.length > 0) {
+      candidate = segments[segments.length - 1] ?? trimmed;
     }
   } catch {
-    // Not a URL — fall through
+    // Not a URL — leave `candidate` as the original input.
   }
-  // Treat as opaque ID; API will surface 404 if it's not numeric.
-  return trimmed;
+
+  // The Smartsheet REST API only accepts numeric IDs. Share-link slugs
+  // look like "V4XgwvVJqFjcxQmQMrgwm77WjCP7hpjqqW82RRR1" (30+ chars,
+  // base64-ish). Detect and reject them with a clear message BEFORE
+  // hitting the API.
+  if (!/^\d+$/.test(candidate)) {
+    throw new Error(
+      "That looks like a Smartsheet share-link slug, not a numeric sheet ID. " +
+        "Open the sheet in Smartsheet (signed in) and copy the numeric ID from the URL — " +
+        "or use File → Properties to find it. " +
+        `Got: "${candidate.slice(0, 12)}${candidate.length > 12 ? "…" : ""}"`,
+    );
+  }
+  return candidate;
 }
 
 /**
@@ -206,14 +225,31 @@ export async function getSheetMeta(
     token,
   );
 
+  // Defensive: Smartsheet occasionally returns a 2xx with a body that
+  // lacks the `result` envelope (e.g. when a non-numeric / share-slug
+  // sheet ID is passed — the API doesn't reject it cleanly). Without
+  // this guard we'd crash with "Cannot read properties of undefined
+  // (reading 'id')" deep in the column-mapping render. `extractSheetId`
+  // catches the common share-slug case up front; this guard is the
+  // belt-and-suspenders for anything else.
+  if (!data?.result || data.result.id == null) {
+    throw new SmartsheetApiError(
+      502,
+      `Smartsheet returned an unexpected response for sheet ${sheetId} ` +
+        `(missing result.id). If you pasted a Smartsheet share-link URL, ` +
+        `paste the numeric sheet ID instead — open the sheet in Smartsheet ` +
+        `while signed in and copy the number from the address bar.`,
+    );
+  }
+
   return {
     id: String(data.result.id),
-    name: data.result.name,
-    columns: data.result.columns.map((c) => ({
-      id: String(c.id),
-      title: c.title,
-      type: c.type,
-      index: c.index,
+    name: data.result.name ?? "(unnamed sheet)",
+    columns: (data.result.columns ?? []).map((c) => ({
+      id: c.id != null ? String(c.id) : "",
+      title: c.title ?? "",
+      type: c.type ?? "TEXT_NUMBER",
+      index: c.index ?? 0,
     })),
   };
 }