diff --git a/pynestml/codegeneration/nest_compartmental_code_generator.py b/pynestml/codegeneration/nest_compartmental_code_generator.py
index 179f3062e..30716efbc 100644
--- a/pynestml/codegeneration/nest_compartmental_code_generator.py
+++ b/pynestml/codegeneration/nest_compartmental_code_generator.py
@@ -27,6 +27,7 @@
 from jinja2 import TemplateRuntimeError
 
 from odetoolbox import analysis
+from pynestml.codegeneration.printers.sympy_simple_expression_printer import SympySimpleExpressionPrinter
 
 import pynestml
 from pynestml.cocos.co_cos_manager import CoCosManager
@@ -180,7 +181,7 @@ def setup_printers(self):
         self._ode_toolbox_variable_printer = ODEToolboxVariablePrinter(None)
         self._ode_toolbox_function_call_printer = ODEToolboxFunctionCallPrinter(None)
         self._ode_toolbox_printer = ODEToolboxExpressionPrinter(
-            simple_expression_printer=CppSimpleExpressionPrinter(
+            simple_expression_printer=SympySimpleExpressionPrinter(
                 variable_printer=self._ode_toolbox_variable_printer,
                 constant_printer=self._constant_printer,
                 function_call_printer=self._ode_toolbox_function_call_printer))
diff --git a/pynestml/codegeneration/printers/gsl_variable_printer.py b/pynestml/codegeneration/printers/gsl_variable_printer.py
index 4de36651a..463833a43 100644
--- a/pynestml/codegeneration/printers/gsl_variable_printer.py
+++ b/pynestml/codegeneration/printers/gsl_variable_printer.py
@@ -18,16 +18,19 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with NEST.  If not, see <http://www.gnu.org/licenses/>.
-
+from pynestml.codegeneration.nest_unit_converter import NESTUnitConverter
 from pynestml.codegeneration.printers.cpp_variable_printer import CppVariablePrinter
 from pynestml.meta_model.ast_variable import ASTVariable
+from pynestml.symbols.predefined_units import PredefinedUnits
 from pynestml.symbols.symbol import SymbolKind
 from pynestml.utils.ast_utils import ASTUtils
+from pynestml.utils.logger import Logger, LoggingLevel
+from pynestml.utils.messages import Messages
 
 
 class GSLVariablePrinter(CppVariablePrinter):
     r"""
-    Reference converter for C++ syntax and using the GSL (GNU Scientific Library) API from inside the ``extern "C"`` stepping function.
+    Variable printer for C++ syntax and using the GSL (GNU Scientific Library) API from inside the ``extern "C"`` stepping function.
     """
 
     def print_variable(self, node: ASTVariable) -> str:
@@ -39,6 +42,16 @@ def print_variable(self, node: ASTVariable) -> str:
         assert isinstance(node, ASTVariable)
         symbol = node.get_scope().resolve_to_symbol(node.get_complete_name(), SymbolKind.VARIABLE)
 
+        if symbol is None:
+            # test if variable name can be resolved to a type
+            if PredefinedUnits.is_unit(node.get_complete_name()):
+                return str(NESTUnitConverter.get_factor(PredefinedUnits.get_unit(node.get_complete_name()).get_unit()))
+
+            code, message = Messages.get_could_not_resolve(node.get_name())
+            Logger.log_message(log_level=LoggingLevel.ERROR, code=code, message=message,
+                               error_position=node.get_source_position())
+            return ""
+
         if node.is_delay_variable():
             return self._print_delay_variable(node)
 
diff --git a/pynestml/codegeneration/printers/sympy_simple_expression_printer.py b/pynestml/codegeneration/printers/sympy_simple_expression_printer.py
index e6a01d465..bf9c64bea 100644
--- a/pynestml/codegeneration/printers/sympy_simple_expression_printer.py
+++ b/pynestml/codegeneration/printers/sympy_simple_expression_printer.py
@@ -20,6 +20,7 @@
 # along with NEST.  If not, see <http://www.gnu.org/licenses/>.
 
 from pynestml.codegeneration.nest_unit_converter import NESTUnitConverter
+from pynestml.codegeneration.printers.cpp_simple_expression_printer import CppSimpleExpressionPrinter
 from pynestml.codegeneration.printers.simple_expression_printer import SimpleExpressionPrinter
 from pynestml.meta_model.ast_function_call import ASTFunctionCall
 from pynestml.meta_model.ast_node import ASTNode
@@ -29,7 +30,7 @@
 from pynestml.symbols.symbol import SymbolKind
 
 
-class SympySimpleExpressionPrinter(SimpleExpressionPrinter):
+class SympySimpleExpressionPrinter(CppSimpleExpressionPrinter):
     r"""
     Printer for ASTSimpleExpressions in Sympy syntax.
     """
diff --git a/pynestml/utils/mechanism_processing.py b/pynestml/utils/mechanism_processing.py
index bc069dbb8..d45257adf 100644
--- a/pynestml/utils/mechanism_processing.py
+++ b/pynestml/utils/mechanism_processing.py
@@ -23,6 +23,8 @@
 
 import copy
 
+from pynestml.codegeneration.printers.sympy_simple_expression_printer import SympySimpleExpressionPrinter
+
 from pynestml.codegeneration.printers.cpp_simple_expression_printer import CppSimpleExpressionPrinter
 from pynestml.codegeneration.printers.nestml_printer import NESTMLPrinter
 from pynestml.codegeneration.printers.constant_printer import ConstantPrinter
@@ -56,7 +58,7 @@ class MechanismProcessing:
     _ode_toolbox_variable_printer = ODEToolboxVariablePrinter(None)
     _ode_toolbox_function_call_printer = ODEToolboxFunctionCallPrinter(None)
     _ode_toolbox_printer = ODEToolboxExpressionPrinter(
-        simple_expression_printer=CppSimpleExpressionPrinter(
+        simple_expression_printer=SympySimpleExpressionPrinter(
             variable_printer=_ode_toolbox_variable_printer,
             constant_printer=_constant_printer,
             function_call_printer=_ode_toolbox_function_call_printer))