diff --git a/FirebasePhoneAuthUI/FUIPhoneEntryViewController.m b/FirebasePhoneAuthUI/FUIPhoneEntryViewController.m
index e416f760578..5d0eb07ea8f 100644
--- a/FirebasePhoneAuthUI/FUIPhoneEntryViewController.m
+++ b/FirebasePhoneAuthUI/FUIPhoneEntryViewController.m
@@ -168,28 +168,36 @@ - (void)onNext:(NSString *)phoneNumber {
   [provider verifyPhoneNumber:phoneNumberWithCountryCode
                    UIDelegate:self
                    completion:^(NSString *_Nullable verificationID, NSError *_Nullable error) {
+    // Temporary fix to guarantee execution of the completion block on the main thread.
+    // TODO: Remove temporary workaround when the issue is fixed in FirebaseAuth.
+    dispatch_block_t completionBlock = ^() {
+      [self decrementActivity];
+      self.navigationItem.rightBarButtonItem.enabled = YES;
+
+      if (error) {
+        [_phoneNumberField becomeFirstResponder];
+
+        UIAlertController *alertController = [FUIPhoneAuth alertControllerForError:error
+                                                                     actionHandler:nil];
+        [self presentViewController:alertController animated:YES completion:nil];
+        
+        FUIPhoneAuth *delegate = [self.authUI providerWithID:FIRPhoneAuthProviderID];
+        [delegate callbackWithCredential:nil error:error result:nil];
+        return;
+      }
 
-    [self decrementActivity];
-    self.navigationItem.rightBarButtonItem.enabled = YES;
-
-    if (error) {
-      [_phoneNumberField becomeFirstResponder];
+      UIViewController *controller =
+          [[FUIPhoneVerificationViewController alloc] initWithAuthUI:self.authUI
+                                                      verificationID:verificationID
+                                                         phoneNumber:phoneNumberWithCountryCode];
 
-      UIAlertController *alertController = [FUIPhoneAuth alertControllerForError:error
-                                                                   actionHandler:nil];
-      [self presentViewController:alertController animated:YES completion:nil];
-      
-      FUIPhoneAuth *delegate = [self.authUI providerWithID:FIRPhoneAuthProviderID];
-      [delegate callbackWithCredential:nil error:error result:nil];
-      return;
+      [self pushViewController:controller];
+    };
+    if ([NSThread isMainThread]) {
+      completionBlock();
+    } else {
+      dispatch_sync(dispatch_get_main_queue(), completionBlock);
     }
-
-    UIViewController *controller =
-        [[FUIPhoneVerificationViewController alloc] initWithAuthUI:self.authUI
-                                                    verificationID:verificationID
-                                                       phoneNumber:phoneNumberWithCountryCode];
-
-    [self pushViewController:controller];
   }];
 }
 
diff --git a/FirebasePhoneAuthUI/FUIPhoneVerificationViewController.m b/FirebasePhoneAuthUI/FUIPhoneVerificationViewController.m
index 8eb632a2751..845fa53262c 100644
--- a/FirebasePhoneAuthUI/FUIPhoneVerificationViewController.m
+++ b/FirebasePhoneAuthUI/FUIPhoneVerificationViewController.m
@@ -138,25 +138,33 @@ - (IBAction)onResendCode:(id)sender {
   [provider verifyPhoneNumber:_phoneNumber
                    UIDelegate:self
                    completion:^(NSString *_Nullable verificationID, NSError *_Nullable error) {
-
-    [self decrementActivity];
-    _verificationID = verificationID;
-    [_codeField becomeFirstResponder];
-
-    if (error) {
-      UIAlertController *alertController = [FUIPhoneAuth alertControllerForError:error
-                                                                   actionHandler:^{
-                                             [_codeField clearCodeInput];
-                                             [_codeField becomeFirstResponder];
-                                           }];
-      [self presentViewController:alertController animated:YES completion:nil];
-      return;
+    // Temporary fix to guarantee execution of the completion block on the main thread.
+    // TODO: Remove temporary workaround when the issue is fixed in FirebaseAuth.
+    dispatch_block_t completionBlock = ^() {
+      [self decrementActivity];
+      _verificationID = verificationID;
+      [_codeField becomeFirstResponder];
+
+      if (error) {
+        UIAlertController *alertController = [FUIPhoneAuth alertControllerForError:error
+                                                                     actionHandler:^{
+                                               [_codeField clearCodeInput];
+                                               [_codeField becomeFirstResponder];
+                                             }];
+        [self presentViewController:alertController animated:YES completion:nil];
+        return;
+      }
+
+      NSString *resultMessage =
+          [NSString stringWithFormat:FUIPhoneAuthLocalizedString(kPAStr_ResendCodeResult),
+              _phoneNumber];
+      [self showAlertWithMessage:resultMessage];
+    };
+    if ([NSThread isMainThread]) {
+      completionBlock();
+    } else {
+      dispatch_sync(dispatch_get_main_queue(), completionBlock);
     }
-
-    NSString *resultMessage =
-        [NSString stringWithFormat:FUIPhoneAuthLocalizedString(kPAStr_ResendCodeResult),
-            _phoneNumber];
-    [self showAlertWithMessage:resultMessage];
   }];
 }
 - (IBAction)onPhoneNumberSelected:(id)sender {
diff --git a/FirebaseUI_dev_auth.podspec b/FirebaseUI_dev_auth.podspec
index 5d622107068..3c310ed2b81 100644
--- a/FirebaseUI_dev_auth.podspec
+++ b/FirebaseUI_dev_auth.podspec
@@ -20,7 +20,7 @@ Pod::Spec.new do |s|
                            'FirebaseAuthUI/**/*.xib']
     }
     authbase.dependency 'Firebase/Analytics'
-    authbase.dependency 'FirebaseAuth'
+    authbase.dependency 'FirebaseAuth', '~> 4.2'
   end
 
 end