"Expected the reducer to be a function." only on server - reactjs

Question

I am building an isomorphic app and I have a strange problem with my store and reducers. When I import my reducers in the client side store, it all works:
import reducers from '../Reducers';
...
let store = createStore(reducers, initial, composeEnhancers(applyMiddleware(...middleware)));
export default store;
But when I try to do the same on the server:
import reducers from '../source/js/Reducers';
I am getting error in the console
Error: Expected the reducer to be a function.
My reducers are like this:
import { INSERT_POST, INSERT_CONTENT, BUMP_PAGE, FETCH_COLLECTION } from '../Actions/Content';
const defaultState = {
currentPage: 0,
nextPage: 1,
content: [],
fetched: []
};
const reducer = (state = defaultState, action) => {
switch (action.type) {
case INSERT_POST:
return { ...state, content: [ ...state.content, action.payload ], fetched: [ ...state.fetched, action.url ] };
case INSERT_CONTENT:
const newState = {...state};
newState.content[action.payload.id].content = action.payload.data;
return newState;
case `${FETCH_COLLECTION}_SUCCESS`:
return { ...state, fetched: [ ...state.fetched, action.meta.previousAction.payload.request.url ]};
case BUMP_PAGE:
return { ...state, currentPage: state.nextPage, nextPage: ++state.nextPage };
default:
return state;
}
};
export default reducer;
...
import { START_TRANSITION, END_TRANSITION, TOGGLE_TRANSITION } from '../Actions/Transitions';
const defaultState = {
loaded: true
};
const reducer = (state = defaultState, action) => {
switch (action.type) {
case START_TRANSITION:
return { ...state, loaded: false };
case END_TRANSITION:
return { ...state, loaded: true };
case TOGGLE_TRANSITION:
return { ...state, loaded: !state.loaded };
default:
return state;
}
};
export default reducer;
And the main reducer:
import { combineReducers } from 'redux';
import Transitions from './Transitions'
import Content from './Content';
export default combineReducers({
Transitions,
Content
});
I have no idea of what to do with this. It's the first time I ever see such error. What can I do?