diff --git a/elasticapm/traces.py b/elasticapm/traces.py
index fb10c7608..bed7e72f5 100644
--- a/elasticapm/traces.py
+++ b/elasticapm/traces.py
@@ -319,9 +319,10 @@ def set_context(data, key="custom"):
         data = data()
 
     # remove invalid characters from key names
-    for k in list(data.keys()):
-        if TAG_RE.search(k):
-            data[TAG_RE.sub("_", k)] = data.pop(k)
+    if not callable(data):  # if transaction wasn't sampled, data is still a callable here and can be ignored
+        for k in list(data.keys()):
+            if TAG_RE.search(k):
+                data[TAG_RE.sub("_", k)] = data.pop(k)
 
     if key in transaction.context:
         transaction.context[key].update(data)
diff --git a/tests/instrumentation/transactions_store_tests.py b/tests/instrumentation/transactions_store_tests.py
index d1ecf0a91..35a757303 100644
--- a/tests/instrumentation/transactions_store_tests.py
+++ b/tests/instrumentation/transactions_store_tests.py
@@ -233,6 +233,17 @@ def test_tags_dedot(elasticapm_client):
     assert transactions[0]["context"]["tags"] == {"d_o_t": "dot", "s_t_a_r": "star", "q_u_o_t_e": "quote"}
 
 
+def test_dedot_is_not_run_when_unsampled(elasticapm_client):
+    for sampled in (True, False):
+        t = elasticapm_client.begin_transaction("test")
+        t.is_sampled = sampled
+        elasticapm.set_context(lambda: {"a.b": "b"})
+        elasticapm_client.end_transaction("x", "OK")
+    sampled_transaction, unsampled_transaction = transactions = elasticapm_client.events[TRANSACTION]
+    assert "a_b" in sampled_transaction["context"]["custom"]
+    assert "context" not in unsampled_transaction
+
+
 def test_set_transaction_name(elasticapm_client):
     elasticapm_client.begin_transaction("test")
     elasticapm_client.end_transaction("test_name", 200)