diff --git a/converter-CellDesigner/src/main/java/lcsb/mapviewer/converter/model/celldesigner/structure/CellDesignerSpecies.java b/converter-CellDesigner/src/main/java/lcsb/mapviewer/converter/model/celldesigner/structure/CellDesignerSpecies.java
index 58a84bd10ab3a616dbdcb15e9c4bb8a40390ce6b..d44dd3c3c385feb88dd8bdbbbcc86cdb16ac10c6 100644
--- a/converter-CellDesigner/src/main/java/lcsb/mapviewer/converter/model/celldesigner/structure/CellDesignerSpecies.java
+++ b/converter-CellDesigner/src/main/java/lcsb/mapviewer/converter/model/celldesigner/structure/CellDesignerSpecies.java
@@ -523,7 +523,7 @@ public class CellDesignerSpecies<T extends Species> extends CellDesignerElement<
         throw new InvalidArgumentException(errorMessage);
       }
     } else {
-      result = null;
+      result = false;
     }
     return result;
   }
diff --git a/converter-CellDesigner/src/test/java/lcsb/mapviewer/converter/model/celldesigner/structure/SpeciesTest.java b/converter-CellDesigner/src/test/java/lcsb/mapviewer/converter/model/celldesigner/structure/SpeciesTest.java
index 733e2a7dbd7089b6aa0859372eea14c5e3e72249..49cd34090f164df7b4149bcf7979757b3a8c4377 100644
--- a/converter-CellDesigner/src/test/java/lcsb/mapviewer/converter/model/celldesigner/structure/SpeciesTest.java
+++ b/converter-CellDesigner/src/test/java/lcsb/mapviewer/converter/model/celldesigner/structure/SpeciesTest.java
@@ -312,8 +312,10 @@ public class SpeciesTest extends CellDesignerTestFunctions {
       }
       species.setOnlySubstanceUnits("false");
       assertFalse(species.getOnlySubstanceUnits());
+      
+      species.setOnlySubstanceUnits("true");
       species.setOnlySubstanceUnits((String) null);
-      assertNull(species.getOnlySubstanceUnits());
+      assertFalse(species.getOnlySubstanceUnits());
 
     } catch (Exception e) {
       e.printStackTrace();
diff --git a/model/src/main/java/lcsb/mapviewer/model/map/species/Species.java b/model/src/main/java/lcsb/mapviewer/model/map/species/Species.java
index 7a396ac047e3fdd95710691958f46c6a45acd54f..35059f3b8600438bea426a7d4acc5fb25a1c6875 100644
--- a/model/src/main/java/lcsb/mapviewer/model/map/species/Species.java
+++ b/model/src/main/java/lcsb/mapviewer/model/map/species/Species.java
@@ -97,7 +97,7 @@ public abstract class Species extends Element {
   /**
    * Is only substance units allowed.
    */
-  private Boolean onlySubstanceUnits = null;
+  private Boolean onlySubstanceUnits = false;
 
   /**
    * How many dimers are in this species.