diff --git a/lshmm/api.py b/lshmm/api.py
index b5c0e10..9d2db7b 100644
--- a/lshmm/api.py
+++ b/lshmm/api.py
@@ -106,13 +106,16 @@ def check_inputs(
         raise ValueError(err_msg)
 
     # Check the recombination probability.
-    if not (
-        isinstance(prob_recombination, (int, float))
-        or (
-            isinstance(prob_recombination, np.ndarray)
-            and len(prob_recombination) == num_sites
-        )
+    if isinstance(prob_recombination, (int, float)):
+        pass
+    elif (
+        isinstance(prob_recombination, np.ndarray)
+        and len(prob_recombination) == num_sites
     ):
+        if prob_recombination[0] != 0:
+            err_msg = "First value in the recombination probability array must be zero."
+            raise ValueError(err_msg)
+    else:
         err_msg = (
             "Recombination probability is not a scalar or an array of expected length."
         )