diff --git a/reasoning_gym/arithmetic/chain_sum.py b/reasoning_gym/arithmetic/chain_sum.py
index 0dccad1f..355bb57b 100644
--- a/reasoning_gym/arithmetic/chain_sum.py
+++ b/reasoning_gym/arithmetic/chain_sum.py
@@ -57,7 +57,7 @@ class ChainSum:
         num_digits = item_rng.randint(self.config.min_digits, self.config.max_digits)
         
         # Calculate value ranges based on number of digits
-        min_value = 10 ** (num_digits - 1)  # e.g., 100 for 3 digits
+        min_value = 0 if num_digits == 1 else 10 ** (num_digits - 1)  # Special case for 1 digit
         max_value = (10 ** num_digits) - 1   # e.g., 999 for 3 digits
         
         expression, result = self._generate_task(item_rng, num_terms, min_value, max_value)
diff --git a/tests/test_chain_sum.py b/tests/test_chain_sum.py
index ff6b0484..b5b0ea98 100644
--- a/tests/test_chain_sum.py
+++ b/tests/test_chain_sum.py
@@ -53,6 +53,7 @@ def test_chain_sum_items():
 
 def test_chain_sum_number_ranges():
     """Test that generated numbers respect digit constraints"""
+    # Test 3-digit numbers
     config = ChainSumConfig(
         min_terms=2,
         max_terms=2,  # Fix to 2 terms for easier testing
@@ -63,6 +64,27 @@ def test_chain_sum_number_ranges():
     )
     dataset = ChainSum(config)
     
+    for i in range(len(dataset)):
+        item = dataset[i]
+        expression = item["metadata"]["expression"]
+        numbers = [int(n) for n in expression.split() if n.isdigit()]
+        for num in numbers:
+            if config.allow_negation:
+                assert -999 <= num <= 999, f"Number {num} outside valid range for 3 digits"
+            else:
+                assert 100 <= num <= 999, f"Number {num} outside valid range for 3 digits"
+
+    # Test 1-digit numbers
+    config = ChainSumConfig(
+        min_terms=2,
+        max_terms=2,
+        min_digits=1,
+        max_digits=1,
+        size=50,
+        seed=42
+    )
+    dataset = ChainSum(config)
+    
     for i in range(len(dataset)):
         item = dataset[i]
         expression = item["metadata"]["expression"]
@@ -76,6 +98,18 @@ def test_chain_sum_number_ranges():
                 assert -999 <= num <= 999, f"Number {num} outside valid range for 3 digits"
             else:
                 assert 100 <= num <= 999, f"Number {num} outside valid range for 3 digits"
+                
+    # Test 1-digit numbers
+    dataset = ChainSum(config)
+    for i in range(len(dataset)):
+        item = dataset[i]
+        expression = item["metadata"]["expression"]
+        numbers = [int(n) for n in expression.split() if n.isdigit()]
+        for num in numbers:
+            if config.allow_negation:
+                assert -9 <= num <= 9, f"Number {num} outside valid range for 1 digit"
+            else:
+                assert 0 <= num <= 9, f"Number {num} outside valid range for 1 digit"
 
 def test_chain_sum_negation():
     """Test that allow_negation controls number ranges"""