diff --git a/epicyon.py b/epicyon.py
index f1abb86d8..7cc492481 100644
--- a/epicyon.py
+++ b/epicyon.py
@@ -83,6 +83,7 @@ from like import sendLikeViaServer
 from like import sendUndoLikeViaServer
 from reaction import sendReactionViaServer
 from reaction import sendUndoReactionViaServer
+from reaction import validEmojiContent
 from skills import sendSkillViaServer
 from availability import setAvailability
 from availability import sendAvailabilityViaServer
@@ -1628,6 +1629,9 @@ if args.react:
     if not args.emoji:
         print('Specify a reaction emoji with the --emoji option')
         sys.exit()
+    if not validEmojiContent(args.emoji):
+        print('This is not a valid emoji')
+        sys.exit()
 
     if not args.password:
         args.password = getpass.getpass('Password: ')
@@ -1698,6 +1702,9 @@ if args.undoreact:
     if not args.emoji:
         print('Specify a reaction emoji with the --emoji option')
         sys.exit()
+    if not validEmojiContent(args.emoji):
+        print('This is not a valid emoji')
+        sys.exit()
 
     if not args.password:
         args.password = getpass.getpass('Password: ')
diff --git a/inbox.py b/inbox.py
index 6469cb839..8d45aeff4 100644
--- a/inbox.py
+++ b/inbox.py
@@ -16,6 +16,7 @@ from linked_data_sig import verifyJsonSignature
 from languages import understoodPostLanguage
 from like import updateLikesCollection
 from reaction import updateReactionCollection
+from reaction import validEmojiContent
 from utils import removeHtml
 from utils import fileLastModified
 from utils import hasObjectString
@@ -1246,6 +1247,10 @@ def _receiveReaction(recentPostsCache: {},
         if debug:
             print('DEBUG: ' + messageJson['type'] + ' content is not string')
         return False
+    if not validEmojiContent(messageJson['content']):
+        print('_receiveReaction: Invalid emoji reaction: "' +
+              messageJson['content'] + '" from ' + messageJson['actor'])
+        return False
     if not hasUsersPath(messageJson['actor']):
         if debug:
             print('DEBUG: "users" or "profile" missing from actor in ' +
diff --git a/reaction.py b/reaction.py
index 2aaea25f6..731fdd37c 100644
--- a/reaction.py
+++ b/reaction.py
@@ -8,6 +8,7 @@ __status__ = "Production"
 __module_group__ = "ActivityPub"
 
 import os
+import re
 from pprint import pprint
 from utils import hasObjectString
 from utils import hasObjectStringObject
@@ -35,6 +36,21 @@ from auth import createBasicAuthHeader
 from posts import getPersonBox
 
 
+emojiRegex = re.compile(r'[\u263a-\U0001f645]')
+
+
+def validEmojiContent(emojiContent: str) -> bool:
+    """Is the given emoji content valid?
+    """
+    if not emojiContent:
+        return False
+    if len(emojiContent) > 1:
+        return False
+    if len(emojiRegex.findall(emojiContent)) == 0:
+        return False
+    return True
+
+
 def noOfReactions(postJsonObject: {}, emojiContent: str) -> int:
     """Returns the number of emoji reactions of a given content type on a post
     """
@@ -75,6 +91,9 @@ def _reaction(recentPostsCache: {},
     """
     if not urlPermitted(objectUrl, federationList):
         return None
+    if not validEmojiContent(emojiContent):
+        print('_reaction: Invalid emoji reaction: "' + emojiContent + '"')
+        return
 
     fullDomain = getFullDomain(domain, port)
 
@@ -179,6 +198,10 @@ def sendReactionViaServer(baseDir: str, session,
     if not session:
         print('WARN: No session for sendReactionViaServer')
         return 6
+    if not validEmojiContent(emojiContent):
+        print('sendReactionViaServer: Invalid emoji reaction: "' +
+              emojiContent + '"')
+        return 7
 
     fromDomainFull = getFullDomain(fromDomain, fromPort)
 
@@ -363,6 +386,10 @@ def outboxReaction(recentPostsCache: {},
         return
     if not isinstance(messageJson['content'], str):
         return
+    if not validEmojiContent(messageJson['content']):
+        print('outboxReaction: Invalid emoji reaction: "' +
+              messageJson['content'] + '"')
+        return
     if debug:
         print('DEBUG: c2s reaction request arrived in outbox')
 
diff --git a/tests.py b/tests.py
index 9a7fb019f..bee2690c4 100644
--- a/tests.py
+++ b/tests.py
@@ -109,6 +109,7 @@ from like import likePost
 from like import sendLikeViaServer
 from reaction import reactionPost
 from reaction import sendReactionViaServer
+from reaction import validEmojiContent
 from announce import announcePublic
 from announce import sendAnnounceViaServer
 from city import parseNogoString
@@ -5909,6 +5910,15 @@ def _testAddCWfromLists(baseDir: str) -> None:
     assert postJsonObject['object']['summary'] == "Murdoch Press / Existing CW"
 
 
+def _testValidEmojiContent() -> None:
+    print('testValidEmojiContent')
+    assert not validEmojiContent(None)
+    assert not validEmojiContent(' ')
+    assert not validEmojiContent('j')
+    assert validEmojiContent('😀')
+    assert validEmojiContent('😄')
+
+
 def runAllTests():
     baseDir = os.getcwd()
     print('Running tests...')
@@ -5916,6 +5926,7 @@ def runAllTests():
     _translateOntology(baseDir)
     _testGetPriceFromString()
     _testFunctions()
+    _testValidEmojiContent()
     _testAddCWfromLists(baseDir)
     _testWordsSimilarity()
     _testSecondsBetweenPublished()