diff --git a/changes.d/612.bugfix b/changes.d/612.bugfix
new file mode 100644
index 000000000..65e80ec0a
--- /dev/null
+++ b/changes.d/612.bugfix
@@ -0,0 +1 @@
+`floor` will now return an integer in lambda expressions with numpy to allow usage in ForLoopPT range expression.
diff --git a/qupulse/utils/sympy.py b/qupulse/utils/sympy.py
index b38d01b16..e27dc9032 100644
--- a/qupulse/utils/sympy.py
+++ b/qupulse/utils/sympy.py
@@ -215,13 +215,30 @@ def numpy_compatible_add(*args) -> Union[sympy.Add, sympy.Array]:
 }
 
 
+def _float_arr_to_int_arr(float_arr):
+    """Try to cast array to int64. Return original array if data is not representable."""
+    int_arr = float_arr.astype(numpy.int64)
+    if numpy.any(int_arr != float_arr):
+        # we either have a float that is too large or NaN
+        return float_arr
+    else:
+        return int_arr
+
+
 def numpy_compatible_ceiling(input_value: Any) -> Any:
     if isinstance(input_value, numpy.ndarray):
-        return numpy.ceil(input_value).astype(numpy.int64)
+        return _float_arr_to_int_arr(numpy.ceil(input_value))
     else:
         return sympy.ceiling(input_value)
 
 
+def _floor_to_int(input_value: Any) -> Any:
+    if isinstance(input_value, numpy.ndarray):
+        return _float_arr_to_int_arr(numpy.floor(input_value))
+    else:
+        return sympy.floor(input_value)
+
+
 def to_numpy(sympy_array: sympy.NDimArray) -> numpy.ndarray:
     if isinstance(sympy_array, sympy.DenseNDimArray):
         if len(sympy_array.shape) == 2:
@@ -327,7 +344,8 @@ def recursive_substitution(expression: sympy.Expr,
 _numpy_environment = {**_base_environment, **numpy.__dict__}
 _sympy_environment = {**_base_environment, **sympy.__dict__}
 
-_lambdify_modules = [{'ceiling': numpy_compatible_ceiling, 'Broadcast': numpy.broadcast_to}, 'numpy', _special_functions]
+_lambdify_modules = [{'ceiling': numpy_compatible_ceiling, 'floor': _floor_to_int,
+                      'Broadcast': numpy.broadcast_to}, 'numpy', _special_functions]
 
 
 def evaluate_compiled(expression: sympy.Expr,
diff --git a/tests/pulses/bug_tests.py b/tests/pulses/bug_tests.py
index fc1aead75..ef4035e9f 100644
--- a/tests/pulses/bug_tests.py
+++ b/tests/pulses/bug_tests.py
@@ -9,6 +9,7 @@
 from qupulse.pulses.repetition_pulse_template import RepetitionPulseTemplate
 from qupulse.pulses.multi_channel_pulse_template import AtomicMultiChannelPulseTemplate
 from qupulse.pulses.mapping_pulse_template import MappingPulseTemplate
+from qupulse.pulses.loop_pulse_template import ForLoopPulseTemplate
 
 from qupulse.pulses.plotting import plot
 
@@ -94,3 +95,12 @@ def test_issue_584_uninitialized_table_sample(self):
 
             expected = np.full_like(times, fill_value=1.)
             np.testing.assert_array_equal(expected, sampled)
+
+    def test_issue_612_for_loop_duration(self):
+        fpt = FunctionPulseTemplate('sin(2*pi*i*t*f)', '1/f')
+        pt = ForLoopPulseTemplate(fpt, 'i', 'floor(total_time*f)')
+        self.assertEqual(
+            (500 + 501) // 2,
+            pt.duration.evaluate_in_scope({'f': 1., 'total_time': 500})
+        )
+