diff --git a/freqtrade/optimize/hyperopt.py b/freqtrade/optimize/hyperopt.py
index 45f6bfb88..d28373c2f 100644
--- a/freqtrade/optimize/hyperopt.py
+++ b/freqtrade/optimize/hyperopt.py
@@ -185,7 +185,8 @@ class Hyperopt:
         if HyperoptTools.has_space(self.config, 'sell'):
             result['sell'] = {p.name: params.get(p.name) for p in self.sell_space}
         if HyperoptTools.has_space(self.config, 'roi'):
-            result['roi'] = self.custom_hyperopt.generate_roi_table(params)
+            result['roi'] = {str(k): v for k, v in
+                             self.custom_hyperopt.generate_roi_table(params).items()}
         if HyperoptTools.has_space(self.config, 'stoploss'):
             result['stoploss'] = {p.name: params.get(p.name) for p in self.stoploss_space}
         if HyperoptTools.has_space(self.config, 'trailing'):
diff --git a/tests/optimize/test_hyperopt.py b/tests/optimize/test_hyperopt.py
index a0c475efb..f23d5bc9e 100644
--- a/tests/optimize/test_hyperopt.py
+++ b/tests/optimize/test_hyperopt.py
@@ -649,10 +649,10 @@ def test_generate_optimizer(mocker, hyperopt_conf) -> None:
                                    'rsi-enabled': False,
                                    'rsi-value': 0,
                                    'trigger': 'macd_cross_signal'},
-                           'roi': {0: 0.12000000000000001,
-                                   20.0: 0.02,
-                                   50.0: 0.01,
-                                   110.0: 0},
+                           'roi': {"0": 0.12000000000000001,
+                                   "20.0": 0.02,
+                                   "50.0": 0.01,
+                                   "110.0": 0},
                            'sell': {'sell-adx-enabled': False,
                                     'sell-adx-value': 0,
                                     'sell-fastd-enabled': True,