diff --git a/python/sdist/amici/sbml_import.py b/python/sdist/amici/sbml_import.py
index be3b007aee..eeb8087563 100644
--- a/python/sdist/amici/sbml_import.py
+++ b/python/sdist/amici/sbml_import.py
@@ -3132,13 +3132,18 @@ def _parse_event_trigger(trigger: sp.Expr) -> sp.Expr:
     Recursively translates a boolean trigger function into a real valued
     root function
 
-    :param trigger:
-    :return: real valued root function expression
+    :param trigger: The Boolean trigger expression.
+        The event triggers when this expression changes from False to True.
+    :return: real-valued root function expression
     """
     # Events can be defined without trigger, i.e., the event will never fire.
     # In this case, set a dummy trigger:
     if trigger is None or trigger is sp.false:
+        return sp.Float(-1.0)
+
+    if trigger is sp.true:
         return sp.Float(1.0)
+
     if trigger.is_Relational:
         root = trigger.args[0] - trigger.args[1]
         _check_unsupported_functions_sbml(root, "sympy.Expression")
diff --git a/python/tests/test_sbml_import.py b/python/tests/test_sbml_import.py
index 3277b233c0..8d2546f3a5 100644
--- a/python/tests/test_sbml_import.py
+++ b/python/tests/test_sbml_import.py
@@ -17,6 +17,7 @@
 from amici import import_model_module
 from amici.testing import TemporaryDirectoryWinSafe as TemporaryDirectory
 from conftest import MODEL_STEADYSTATE_SCALED_XML
+import sympy as sp
 
 
 def simple_sbml_model():
@@ -41,6 +42,22 @@ def simple_sbml_model():
     return document, model
 
 
+def test_event_trigger_to_root_function():
+    """Test that root functions for event triggers are generated correctly."""
+    from amici.sbml_import import _parse_event_trigger as to_trig
+
+    a, b = sp.symbols("a b")
+
+    assert to_trig(None) == sp.Float(-1)
+    assert to_trig(sp.false) == sp.Float(-1)
+    assert to_trig(sp.true) == sp.Float(1)
+
+    assert to_trig(a > b) == a - b
+    assert to_trig(a >= b) == a - b
+    assert to_trig(a < b) == b - a
+    assert to_trig(a <= b) == b - a
+
+
 def test_sbml2amici_no_observables(tempdir):
     """Test model generation works for model without observables"""
     sbml_doc, sbml_model = simple_sbml_model()