Fix MissingDenominationError

diff --git a/units/src/amount/error.rs b/units/src/amount/error.rs
index 7f098334..1f4b23ba 100644
--- a/units/src/amount/error.rs
+++ b/units/src/amount/error.rs
@@ -39,8 +39,7 @@ impl fmt::Display for ParseError {
             ParseErrorInner::Amount(ref e) => write_err!(f, "invalid amount"; e),
             ParseErrorInner::Denomination(ref e) => write_err!(f, "invalid denomination"; e),
             // We consider this to not be a source because it currently doesn't contain useful info.
-            ParseErrorInner::MissingDenomination(_) =>
-                f.write_str("the input doesn't contain a denomination"),
+            ParseErrorInner::MissingDenomination(ref e) => write_err!(f, "missing denomination"; e),
         }
     }
 }
@@ -52,7 +51,7 @@ impl std::error::Error for ParseError {
             ParseErrorInner::Amount(ref e) => Some(e),
             ParseErrorInner::Denomination(ref e) => Some(e),
             // We consider this to not be a source because it currently doesn't contain useful info.
-            ParseErrorInner::MissingDenomination(_) => None,
+            ParseErrorInner::MissingDenomination(ref e) => Some(e),
         }
     }
 }
@@ -375,6 +374,17 @@ impl From<Infallible> for MissingDenominationError {
     fn from(never: Infallible) -> Self { match never {} }
 }
 
+impl fmt::Display for MissingDenominationError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "the input does not contain a denomination")
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for MissingDenominationError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
+
 /// Error returned when parsing an unknown denomination.
 #[derive(Debug, Clone, PartialEq, Eq)]
 #[non_exhaustive]
@@ -618,7 +628,7 @@ mod tests {
         let e = "123".parse::<Amount>().unwrap_err();
         assert!(!e.to_string().is_empty());
         #[cfg(feature = "std")]
-        assert!(e.source().is_none());
+        assert!(e.source().is_some());
 
         #[cfg(feature = "encoding")]
         {
diff --git a/units/src/locktime/relative/error.rs b/units/src/locktime/relative/error.rs
index c8243e6e..9ab5daee 100644
--- a/units/src/locktime/relative/error.rs
+++ b/units/src/locktime/relative/error.rs
@@ -14,10 +14,6 @@ use super::{NumberOf512Seconds, NumberOfBlocks};
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct DisabledLockTimeError(pub(super) u32);
 
-impl From<Infallible> for DisabledLockTimeError {
-    fn from(never: Infallible) -> Self { match never {} }
-}
-
 impl DisabledLockTimeError {
     /// Accessor for the `u32` whose "disable" flag was set, preventing
     /// it from being parsed as a relative locktime.
@@ -25,6 +21,10 @@ impl DisabledLockTimeError {
     pub fn disabled_locktime_value(&self) -> u32 { self.0 }
 }
 
+impl From<Infallible> for DisabledLockTimeError {
+    fn from(never: Infallible) -> Self { match never {} }
+}
+
 impl fmt::Display for DisabledLockTimeError {
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {