diff --git a/src/key/verification/start.rs b/src/key/verification/start.rs
index be40dc86..f7c4c019 100644
--- a/src/key/verification/start.rs
+++ b/src/key/verification/start.rs
@@ -52,10 +52,15 @@ impl<'de> Deserialize<'de> for EventResult<StartEvent> {
             }
         };
 
-        let content = match raw.content {
-            raw::StartEventContent::MSasV1(content) => StartEventContent::MSasV1(content),
-            raw::StartEventContent::__Nonexhaustive => {
-                panic!("__Nonexhaustive enum variant is not intended for use.");
+        let content = match StartEventContent::from_raw(raw.content) {
+            Ok(content) => content,
+            Err(error) => {
+                return Ok(EventResult::Err(InvalidEvent(
+                    InnerInvalidEvent::Validation {
+                        json,
+                        message: error.to_string(),
+                    },
+                )));
             }
         };
 
@@ -123,6 +128,44 @@ impl_event!(
     EventType::KeyVerificationStart
 );
 
+impl StartEventContent {
+    fn from_raw(raw: raw::StartEventContent) -> Result<Self, &'static str> {
+        match raw {
+            raw::StartEventContent::MSasV1(content) => {
+                if !content
+                    .key_agreement_protocols
+                    .contains(&KeyAgreementProtocol::Curve25519)
+                {
+                    return Err("`key_agreement_protocols` must contain at least `KeyAgreementProtocol::Curve25519`");
+                }
+
+                if !content.hashes.contains(&HashAlgorithm::Sha256) {
+                    return Err("`hashes` must contain at least `HashAlgorithm::Sha256`");
+                }
+
+                if !content
+                    .message_authentication_codes
+                    .contains(&MessageAuthenticationCode::HkdfHmacSha256)
+                {
+                    return Err("`message_authentication_codes` must contain at least `MessageAuthenticationCode::HkdfHmacSha256`");
+                }
+
+                if !content
+                    .short_authentication_string
+                    .contains(&ShortAuthenticationString::Decimal)
+                {
+                    return Err("`short_authentication_string` must contain at least `ShortAuthenticationString::Decimal`");
+                }
+
+                Ok(StartEventContent::MSasV1(content))
+            }
+            raw::StartEventContent::__Nonexhaustive => {
+                panic!("__Nonexhaustive enum variant is not intended for use.");
+            }
+        }
+    }
+}
+
 impl<'de> Deserialize<'de> for EventResult<StartEventContent> {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
@@ -142,53 +185,14 @@ impl<'de> Deserialize<'de> for EventResult<StartEventContent> {
             }
         };
 
-        match raw {
-            raw::StartEventContent::MSasV1(content) => {
-                if !content
-                    .key_agreement_protocols
-                    .contains(&KeyAgreementProtocol::Curve25519)
-                {
-                    return Ok(EventResult::Err(InvalidEvent(InnerInvalidEvent::Validation {
-                        json,
-                        message: "`key_agreement_protocols` must contain at least `KeyAgreementProtocol::Curve25519`".to_string(),
-                    })));
-                }
-
-                if !content.hashes.contains(&HashAlgorithm::Sha256) {
-                    return Ok(EventResult::Err(InvalidEvent(
-                        InnerInvalidEvent::Validation {
-                            json,
-                            message: "`hashes` must contain at least `HashAlgorithm::Sha256`"
-                                .to_string(),
-                        },
-                    )));
-                }
-
-                if !content
-                    .message_authentication_codes
-                    .contains(&MessageAuthenticationCode::HkdfHmacSha256)
-                {
-                    return Ok(EventResult::Err(InvalidEvent(InnerInvalidEvent::Validation {
-                        json,
-                        message: "`message_authentication_codes` must contain at least `MessageAuthenticationCode::HkdfHmacSha256`".to_string(),
-                    })));
-                }
-
-                if !content
-                    .short_authentication_string
-                    .contains(&ShortAuthenticationString::Decimal)
-                {
-                    return Ok(EventResult::Err(InvalidEvent(InnerInvalidEvent::Validation {
-                        json,
-                        message: "`short_authentication_string` must contain at least `ShortAuthenticationString::Decimal`".to_string(),
-                    })));
-                }
-
-                Ok(EventResult::Ok(StartEventContent::MSasV1(content)))
-            }
-            raw::StartEventContent::__Nonexhaustive => {
-                panic!("__Nonexhaustive enum variant is not intended for use.");
-            }
+        match StartEventContent::from_raw(raw) {
+            Ok(content) => Ok(EventResult::Ok(content)),
+            Err(error) => Ok(EventResult::Err(InvalidEvent(
+                InnerInvalidEvent::Validation {
+                    json,
+                    message: error.to_string(),
+                },
+            ))),
         }
     }
 }
@@ -696,4 +700,19 @@ mod tests {
         assert!(error.message().contains("short_authentication_string"));
         assert!(error.json().is_some());
     }
+
+    #[test]
+    fn deserialization_of_event_validates_content() {
+        // This JSON is missing the required value of "curve25519" for "key_agreement_protocols".
+        let error =
+            serde_json::from_str::<EventResult<StartEvent>>(
+                r#"{"content":{"from_device":"123","transaction_id":"456","method":"m.sas.v1","key_agreement_protocols":[],"hashes":["sha256"],"message_authentication_codes":["hkdf-hmac-sha256"],"short_authentication_string":["decimal"]},"type":"m.key.verification.start"}"#
+            )
+                .unwrap()
+                .into_result()
+                .unwrap_err();
+
+        assert!(error.message().contains("key_agreement_protocols"));
+        assert!(error.json().is_some());
+    }
 }