diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 873062c8..cbf83df4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -116,7 +116,7 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { rust: nightly, os: ubuntu-latest }
+          - { rust: nightly-2020-09-21, os: ubuntu-latest }
     steps:
       - name: Checkout
         uses: actions/checkout@v2
diff --git a/src/registry/stringify_exec_doc.rs b/src/registry/stringify_exec_doc.rs
index 5fd2b554..7e7278d0 100644
--- a/src/registry/stringify_exec_doc.rs
+++ b/src/registry/stringify_exec_doc.rs
@@ -46,9 +46,9 @@ impl Registry {
                             "{}: {}",
                             variable_definition.node.name.node,
                             variable_definition.node.var_type.node
-                        );
+                        )?;
                         if let Some(default_value) = &variable_definition.node.default_value {
-                            write!(output, " = {}", default_value.node);
+                            write!(output, " = {}", default_value.node)?;
                         }
                     }
                     output.push(')');