diff --git a/src/cargo/ops/cargo_new.rs b/src/cargo/ops/cargo_new.rs
index 4cdd74d5ee3e..b78ba243e3eb 100644
--- a/src/cargo/ops/cargo_new.rs
+++ b/src/cargo/ops/cargo_new.rs
@@ -1022,7 +1022,7 @@ fn update_manifest_with_new_member(
                 }
             }
         }
-    } else {
+    } else if workspace_document.contains_key("workspace") {
         let ws = workspace_document["workspace"]
             .as_table_mut()
             .with_context(|| format!("[workspace] is not a table"))?;
@@ -1050,6 +1050,11 @@ fn update_manifest_with_new_member(
         }
 
         ws.insert("members", toml_edit::value(array));
+    } else {
+        let mut array = Array::new();
+        array.push(relpath);
+        array.decor_mut().set_suffix("\n");
+        workspace_document["workspace"]["members"] = toml_edit::value(array);
     }
 
     paths::write(&root_manifest_path, workspace_document.to_string())