diff --git a/src/lib.rs b/src/lib.rs
index 347b937..c71afd7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -337,6 +337,12 @@
     ///
     /// This method is available when building with a nightly compiler, or when
     /// building with rustc 1.29+ *without* semver exempt features.
+    ///
+    /// # Panics
+    ///
+    /// Panics if called from outside of a procedural macro. Unlike
+    /// `proc_macro2::Span`, the `proc_macro::Span` type can only exist within
+    /// the context of a procedural macro invocation.
     #[cfg(wrap_proc_macro)]
     pub fn unstable(self) -> proc_macro::Span {
         self.inner.unstable()
diff --git a/src/unstable.rs b/src/unstable.rs
index c4cf11a..57488dd 100644
--- a/src/unstable.rs
+++ b/src/unstable.rs
@@ -473,7 +473,9 @@
     pub fn unstable(self) -> proc_macro::Span {
         match self {
             Span::Nightly(s) => s,
-            Span::Stable(_) => mismatch(),
+            Span::Stable(_) => {
+                panic!("proc_macro::Span is only available in procedural macros")
+            }
         }
     }