diff --git a/hl7/containers.py b/hl7/containers.py
index 1b1626b..73ace74 100644
--- a/hl7/containers.py
+++ b/hl7/containers.py
@@ -742,12 +742,26 @@ def assign_field(
         while len(field) < repeat_num:
             field.append(self.create_repetition([]))
         repetition = field(repeat_num)
+        if not isinstance(repetition, Repetition):
+            # A non-repeating field stores its value as a bare string. Promote
+            # it to a Repetition so it can be addressed at the component or
+            # subcomponent level instead of raising a TypeError.
+            repetition = self.create_repetition(
+                [repetition] if repetition != "" else []
+            )
+            field(repeat_num, repetition)
         if component_num is None:
             repetition[:] = [value]
             return
         while len(repetition) < component_num:
             repetition.append(self.create_component([]))
         component = repetition(component_num)
+        if not isinstance(component, Component):
+            # Likewise, a single-component repetition stores a bare string.
+            component = self.create_component(
+                [component] if component != "" else []
+            )
+            repetition(component_num, component)
         if subcomponent_num is None:
             component[:] = [value]
             return
diff --git a/tests/test_parse.py b/tests/test_parse.py
index 39cc786..faa0a25 100644
--- a/tests/test_parse.py
+++ b/tests/test_parse.py
@@ -356,6 +356,27 @@ def test_assign(self):
         self.assertEqual(msg2["MSH.21.1.1"], "COMPONENT 21.1.1")
         self.assertEqual(msg2["MSH.21.1.2.4"], "SUBCOMPONENT 21.1.2.4")
 
+    def test_assign_deeper_into_simple_field(self):
+        # Regression for #55: assigning into a field/repetition that was parsed
+        # as a bare string must promote it to a container rather than raising
+        # "TypeError: 'str' object does not support item assignment".
+        sample = "MSH|^~\\&|field|rep1~rep2|"
+
+        # repetition level (the reported case)
+        seg = hl7.parse(sample).segments("MSH")[0]
+        seg.assign_field("NewRep", 3, 1)
+        self.assertEqual(str(seg(3)), "NewRep")
+
+        # component level keeps the existing value as the first component
+        seg = hl7.parse(sample).segments("MSH")[0]
+        seg.assign_field("X", 3, 1, 2)
+        self.assertEqual(str(seg(3)), "field^X")
+
+        # subcomponent level into a single-component repetition
+        seg = hl7.parse(sample).segments("MSH")[0]
+        seg.assign_field("Y", 4, 1, 1, 2)
+        self.assertEqual(str(seg(4)), "rep1&Y~rep2")
+
     def test_unescape(self):
         msg = hl7.parse(rep_sample_hl7)