diff --git a/pde/solvers/explicit_mpi.py b/pde/solvers/explicit_mpi.py
index 16b4cb9d..a79ec2bb 100644
--- a/pde/solvers/explicit_mpi.py
+++ b/pde/solvers/explicit_mpi.py
@@ -121,12 +121,14 @@ def _make_error_synchronizer(self) -> Callable[[float], float]:
         """return helper function that synchronizes errors between multiple processes"""
         if mpi.parallel_run:
             # in a parallel run, we need to return the maximal error
-            from ..tools.mpi import mpi_allreduce
+            from ..tools.mpi import Operator, mpi_allreduce
+
+            operator_max_id = Operator.MAX
 
             @register_jitable
             def synchronize_errors(error: float) -> float:
                 """return maximal error accross all cores"""
-                return mpi_allreduce(error, "MAX")  # type: ignore
+                return mpi_allreduce(error, operator_max_id)  # type: ignore
 
             return synchronize_errors  # type: ignore
         else:
diff --git a/pde/tools/mpi.py b/pde/tools/mpi.py
index 6538c9f6..eb6e37fe 100644
--- a/pde/tools/mpi.py
+++ b/pde/tools/mpi.py
@@ -27,6 +27,11 @@
 from numba import types
 from numba.extending import overload, register_jitable
 
+try:
+    from numba.types import Literal
+except ImportError:
+    from numba.types.misc import Literal
+
 if TYPE_CHECKING:
     from numba_mpi import Operator
 
@@ -184,18 +189,26 @@ def ol_mpi_allreduce(data, operator: int | str | None = None):
 
     if operator is None or isinstance(operator, nb.types.NoneType):
         op_id = -1  # value will not be used
-    elif isinstance(operator, nb.types.misc.StringLiteral):
-        op_id = Operator.id(operator.literal_value)
-    elif isinstance(operator, nb.types.misc.Literal):
-        op_id = int(operator)
+    elif isinstance(operator, Literal):
+        # an operator is specified (using a literal value
+        if isinstance(operator.literal_value, str):
+            # an operator is specified by it's name
+            op_id = Operator.id(operator.literal_value)
+        else:
+            # assume an operator is specified by it's id
+            op_id = int(operator.literal_value)
+    elif isinstance(operator, nb.types.Integer):
+        op_id = None  # use given value of operator
     else:
-        raise RuntimeError("`operator` must be a literal type")
+        raise RuntimeError(f"`operator` must be a literal type, not {operator}")
 
     @register_jitable
     def _allreduce(sendobj, recvobj, operator: int | str | None = None) -> int:
         """helper function that calls `numba_mpi.allreduce`"""
         if operator is None:
             return numba_mpi.allreduce(sendobj, recvobj)  # type: ignore
+        elif op_id is None:
+            return numba_mpi.allreduce(sendobj, recvobj, operator)  # type: ignore
         else:
             return numba_mpi.allreduce(sendobj, recvobj, op_id)  # type: ignore
 
diff --git a/pde/tools/resources/requirements_basic.txt b/pde/tools/resources/requirements_basic.txt
index 9d406830..01982929 100644
--- a/pde/tools/resources/requirements_basic.txt
+++ b/pde/tools/resources/requirements_basic.txt
@@ -1,7 +1,7 @@
 # These are the basic requirements for the package
 matplotlib>=3.1
 numba>=0.59
-numpy>=1.22
+numpy>=1.22,<2
 scipy>=1.10
 sympy>=1.9
 tqdm>=4.66
diff --git a/pde/tools/resources/requirements_full.txt b/pde/tools/resources/requirements_full.txt
index 16bab919..54881188 100644
--- a/pde/tools/resources/requirements_full.txt
+++ b/pde/tools/resources/requirements_full.txt
@@ -3,7 +3,7 @@ ffmpeg-python>=0.2
 h5py>=2.10
 matplotlib>=3.1
 numba>=0.59
-numpy>=1.22
+numpy>=1.22,<2
 pandas>=2
 py-modelrunner>=0.18
 rocket-fft>=0.2.4
diff --git a/pde/tools/resources/requirements_mpi.txt b/pde/tools/resources/requirements_mpi.txt
index af4b800c..5aa6a093 100644
--- a/pde/tools/resources/requirements_mpi.txt
+++ b/pde/tools/resources/requirements_mpi.txt
@@ -4,7 +4,7 @@ matplotlib>=3.1
 mpi4py>=3
 numba>=0.59
 numba-mpi>=0.22
-numpy>=1.22
+numpy>=1.22,<2
 pandas>=2
 scipy>=1.10
 sympy>=1.9
diff --git a/pyproject.toml b/pyproject.toml
index 0aeb1801..bd22625c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -27,7 +27,7 @@ classifiers = [
 ]
 
 # Requirements for setuptools
-dependencies = ["matplotlib>=3.1", "numba>=0.59", "numpy>=1.22", "scipy>=1.10", "sympy>=1.9", "tqdm>=4.66"]
+dependencies = ["matplotlib>=3.1", "numba>=0.59", "numpy>=1.22,<2", "scipy>=1.10", "sympy>=1.9", "tqdm>=4.66"]
 
 [project.optional-dependencies]
 io = ["h5py>=2.10", "pandas>=2", "ffmpeg-python>=0.2"]
diff --git a/requirements.txt b/requirements.txt
index a043079f..c634cc4a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,6 @@
 matplotlib>=3.1
 numba>=0.59
-numpy>=1.22
+numpy>=1.22,<2
 scipy>=1.10
 sympy>=1.9
 tqdm>=4.66
diff --git a/scripts/create_requirements.py b/scripts/create_requirements.py
index ce35c6f1..2eccc5fd 100755
--- a/scripts/create_requirements.py
+++ b/scripts/create_requirements.py
@@ -72,7 +72,7 @@ def line(self, relation: str = ">=") -> str:
     ),
     Requirement(
         name="numpy",
-        version_min="1.22",
+        version_min="1.22,<2",
         usage="Handling numerical data",
         essential=True,
     ),
diff --git a/tests/requirements_full.txt b/tests/requirements_full.txt
index 16bab919..54881188 100644
--- a/tests/requirements_full.txt
+++ b/tests/requirements_full.txt
@@ -3,7 +3,7 @@ ffmpeg-python>=0.2
 h5py>=2.10
 matplotlib>=3.1
 numba>=0.59
-numpy>=1.22
+numpy>=1.22,<2
 pandas>=2
 py-modelrunner>=0.18
 rocket-fft>=0.2.4
diff --git a/tests/requirements_min.txt b/tests/requirements_min.txt
index fde8f671..64383746 100644
--- a/tests/requirements_min.txt
+++ b/tests/requirements_min.txt
@@ -1,7 +1,7 @@
 # These are the minimal requirements used to test compatibility
 matplotlib~=3.1
 numba~=0.59
-numpy~=1.22
+numpy~=1.22,<2
 scipy~=1.10
 sympy~=1.9
 tqdm~=4.66
diff --git a/tests/requirements_mpi.txt b/tests/requirements_mpi.txt
index 82ab6622..b53de48f 100644
--- a/tests/requirements_mpi.txt
+++ b/tests/requirements_mpi.txt
@@ -4,7 +4,7 @@ matplotlib>=3.1
 mpi4py>=3
 numba>=0.59
 numba-mpi>=0.22
-numpy>=1.22
+numpy>=1.22,<2
 pandas>=2
 scipy>=1.10
 sympy>=1.9