diff --git a/Signal/src/Profiles/OWSProfileManager.h b/Signal/src/Profiles/OWSProfileManager.h
index 175371572..8a7bd3e74 100644
--- a/Signal/src/Profiles/OWSProfileManager.h
+++ b/Signal/src/Profiles/OWSProfileManager.h
@@ -14,6 +14,7 @@ extern NSString *const kNSNotificationKey_ProfileRecipientId;
 extern NSString *const kNSNotificationKey_ProfileGroupId;
 
 extern const NSUInteger kOWSProfileManager_MaxAvatarDiameter;
+extern const NSUInteger kOWSProfileManager_NameDataLength;
 
 @class TSThread;
 @class OWSAES256Key;
diff --git a/Signal/src/Profiles/OWSProfileManager.m b/Signal/src/Profiles/OWSProfileManager.m
index 5ef2fac70..822fdd627 100644
--- a/Signal/src/Profiles/OWSProfileManager.m
+++ b/Signal/src/Profiles/OWSProfileManager.m
@@ -99,7 +99,7 @@ NSString *const kOWSProfileManager_GroupWhitelistCollection = @"kOWSProfileManag
 
 // The max bytes for a user's profile name, encoded in UTF8.
 // Before encrypting and submitting we NULL pad the name data to this length.
-static const NSUInteger kOWSProfileManager_NameDataLength = 26;
+const NSUInteger kOWSProfileManager_NameDataLength = 26;
 const NSUInteger kOWSProfileManager_MaxAvatarDiameter = 640;
 
 @interface OWSProfileManager ()
diff --git a/Signal/src/ViewControllers/ProfileViewController.m b/Signal/src/ViewControllers/ProfileViewController.m
index d9f820351..8d09af086 100644
--- a/Signal/src/ViewControllers/ProfileViewController.m
+++ b/Signal/src/ViewControllers/ProfileViewController.m
@@ -448,12 +448,52 @@ NSString *const kProfileView_LastPresentedDate = @"kProfileView_LastPresentedDat
 #pragma mark - UITextFieldDelegate
 
 - (BOOL)textField:(UITextField *)textField
-    shouldChangeCharactersInRange:(NSRange)range
+    shouldChangeCharactersInRange:(NSRange)editingRange
                 replacementString:(NSString *)insertionText
 {
     // TODO: Possibly filter invalid input.
-    // TODO: Possibly prevent user from typing overlong name.
-    return YES;
+
+    // Prevent crashing undo bug
+    if (editingRange.length + editingRange.location > textField.text.length) {
+        return NO;
+    }
+
+    NSUInteger (^byteLength)(NSString *) = ^NSUInteger(NSString *string) {
+        if (string == nil) {
+            return 0;
+        }
+
+        return [string dataUsingEncoding:NSUTF8StringEncoding].length;
+    };
+
+    NSUInteger lengthOfRemainingExistingString
+        = byteLength(textField.text) - byteLength([textField.text substringWithRange:editingRange]);
+    NSUInteger newLength = lengthOfRemainingExistingString + byteLength(insertionText);
+
+    DDLogVerbose(@"%@ newLength: %lu", self.tag, (unsigned long)newLength);
+    if (newLength <= kOWSProfileManager_NameDataLength) {
+        return YES;
+    }
+
+    // Don't allow any change if inserting a single char (typically this means typing)
+    if (insertionText.length < 2) {
+        return NO;
+    }
+
+    // However if pasting, accept as much of the string as possible.
+    NSUInteger availableSpace = kOWSProfileManager_NameDataLength - lengthOfRemainingExistingString;
+
+    NSString *acceptableSubstring = @"";
+    for (NSUInteger i = 0; i <= insertionText.length; i++) {
+        NSString *maybeAcceptableSubstring = [insertionText substringWithRange:NSMakeRange(0, i)];
+        if (byteLength(maybeAcceptableSubstring) <= availableSpace) {
+            acceptableSubstring = maybeAcceptableSubstring;
+        }
+    }
+    textField.text = [textField.text stringByReplacingCharactersInRange:editingRange withString:acceptableSubstring];
+
+    // We've already handled any valid editing manually, so prevent further changes.
+    return NO;
 }
 
 - (BOOL)textFieldShouldReturn:(UITextField *)textField