diff --git a/format.cpp b/format.cpp
index 284e3d3a..78104472 100644
--- a/format.cpp
+++ b/format.cpp
@@ -17,10 +17,3 @@
 #include "format.hpp"
 
 #include <utility>
-
-
-//std::string
-//util::format (std::string &&fmt)
-//{
-//    return std::move (fmt);
-//}
diff --git a/format.hpp b/format.hpp
index 31f6ddff..3947a18e 100644
--- a/format.hpp
+++ b/format.hpp
@@ -11,7 +11,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  *
- * Copyright 2015 Danny Robson <danny@nerdcruft.net>
+ * Copyright 2015-2016 Danny Robson <danny@nerdcruft.net>
  */
 
 #ifndef __UTIL_FORMAT_HPP
@@ -37,6 +37,19 @@ namespace util {
         class format_error : public error
         { using error::error; };
 
+        template <typename ValueT>
+        class invalid_specifier : public format_error {
+        public:
+            using value_type = ValueT;
+
+            invalid_specifier (char specifier);
+
+            char specifier (void) const;
+
+        private:
+            char m_specifier;
+        };
+
         // missing format specifier
         class missing_error : public error
         { using error::error; };
diff --git a/format.ipp b/format.ipp
index ea2a1411..7e6afcc4 100644
--- a/format.ipp
+++ b/format.ipp
@@ -165,7 +165,7 @@ namespace util {
                 throw util::format::format_error ("missing format specifier");
 
             if (!is_valid_specifier<typename std::decay<ValueT>::type> (&*spec))
-                throw util::format::format_error ("invalid/unhandled format specifier");
+                throw util::format::invalid_specifier<ValueT> (*spec);
 
             if (*spec == 'x') {
                 dest << std::hex << val << std::dec;
@@ -194,4 +194,25 @@ namespace util {
             return out.str ();
         }
     }
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    // TODO: we'd like to use typeid here for type naming, but we don't allow
+    // RTTI. revisit this when introspection is more advanced.
+    template <typename ValueT>
+    format::invalid_specifier<ValueT>::invalid_specifier (char _specifier):
+        format_error (
+            format::render ("invalid specifier '%c' for type '%s'",
+                            _specifier,
+                            "unimplemented")
+        ),
+        m_specifier (_specifier)
+    { ; }
+
+
+    //-------------------------------------------------------------------------
+    template <typename ValueT>
+    char
+    format::invalid_specifier<ValueT>::specifier (void) const
+    { return m_specifier; }
 }
diff --git a/test/format.cpp b/test/format.cpp
index a3c0cb37..c3aa6fb1 100644
--- a/test/format.cpp
+++ b/test/format.cpp
@@ -17,7 +17,7 @@ main (void)
         util::format::render ("%s");
     }, "missing value");
 
-    tap.expect_throw<util::format::format_error> ([] (void) {
+    tap.expect_throw<util::format::invalid_specifier<int>> ([] (void) {
         util::format::render ("%<", 42);
     }, "invalid specifier");