diff --git a/Bugsnag/Delivery/BSGEventUploadOperation.m b/Bugsnag/Delivery/BSGEventUploadOperation.m
index 2205ac191..73af4a06d 100644
--- a/Bugsnag/Delivery/BSGEventUploadOperation.m
+++ b/Bugsnag/Delivery/BSGEventUploadOperation.m
@@ -87,8 +87,16 @@ - (void)runWithDelegate:(id<BSGEventUploadOperationDelegate>)delegate completion
         }
     }
     
-    NSDictionary *eventPayload = [event toJsonWithRedactedKeys:configuration.redactedKeys];
-
+    NSDictionary *eventPayload;
+    @try {
+        eventPayload = [event toJsonWithRedactedKeys:configuration.redactedKeys];
+    } @catch (NSException *exception) {
+        bsg_log_err(@"Discarding event %@ because an exception was thrown by -toJsonWithRedactedKeys: %@", self.name, exception);
+        [self deleteEvent];
+        completionHandler();
+        return;
+    }
+    
     NSString *apiKey = event.apiKey ?: configuration.apiKey;
     
     NSMutableDictionary *requestPayload = [NSMutableDictionary dictionary];
diff --git a/Bugsnag/Payload/BugsnagEvent.m b/Bugsnag/Payload/BugsnagEvent.m
index 51af6c3bb..b423ab18d 100644
--- a/Bugsnag/Payload/BugsnagEvent.m
+++ b/Bugsnag/Payload/BugsnagEvent.m
@@ -563,7 +563,11 @@ - (NSDictionary *)toJsonWithRedactedKeys:(NSSet *)redactedKeys {
 
     // add metadata
     NSMutableDictionary *metadata = [[[self metadata] toDictionary] mutableCopy];
-    event[BSGKeyMetadata] = [self sanitiseMetadata:metadata redactedKeys:redactedKeys];
+    @try {
+        event[BSGKeyMetadata] = [self sanitiseMetadata:metadata redactedKeys:redactedKeys];
+    } @catch (NSException *exception) {
+        bsg_log_err(@"An exception was thrown while sanitising metadata: %@", exception);
+    }
 
     event[BSGKeyDevice] = [self.device toDictionary];
     event[BSGKeyApp] = [self.app toDict];
@@ -605,7 +609,15 @@ - (NSDictionary *)toJsonWithRedactedKeys:(NSSet *)redactedKeys {
 
 - (NSMutableDictionary *)sanitiseMetadata:(NSMutableDictionary *)metadata redactedKeys:(NSSet *)redactedKeys {
     for (NSString *sectionKey in [metadata allKeys]) {
-        metadata[sectionKey] = [metadata[sectionKey] mutableCopy];
+        if ([metadata[sectionKey] isKindOfClass:[NSDictionary class]]) {
+            metadata[sectionKey] = [metadata[sectionKey] mutableCopy];
+        } else {
+            NSString *message = [NSString stringWithFormat:@"Expected an NSDictionary but got %@ %@",
+                                 NSStringFromClass([metadata[sectionKey] class]), metadata[sectionKey]];
+            bsg_log_err(@"%@", message);
+            // Leave an indication of the error in the payload for diagnosis
+            metadata[sectionKey] = [@{@"bugsnag.error": message} mutableCopy];
+        }
         NSMutableDictionary *section = metadata[sectionKey];
 
         if (section != nil) { // redact sensitive metadata values
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9b5dca038..e6c67a603 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,9 @@ Changelog
 
 ### Bug fixes
 
+* Catch exceptions thrown while preparing JSON for upload rather than crashing.
+  [#1063](https://github.com/bugsnag/bugsnag-cocoa/pull/1063)
+
 * Prevent app hangs being reported if a debugger is attached.
   [#1058](https://github.com/bugsnag/bugsnag-cocoa/pull/1058)