diff --git a/src/FDM/JSBSim/FGTank.cpp b/src/FDM/JSBSim/FGTank.cpp
index dd4407317..83d85744f 100644
--- a/src/FDM/JSBSim/FGTank.cpp
+++ b/src/FDM/JSBSim/FGTank.cpp
@@ -54,7 +54,7 @@ CLASS IMPLEMENTATION
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
 
 FGTank::FGTank(FGConfigFile* AC_cfg)
-    : Contents(0.0)
+    : Contents(-1.0)
 {
   string token;
   
@@ -72,7 +72,7 @@ FGTank::FGTank(FGConfigFile* AC_cfg)
     else if (token == "RADIUS") *AC_cfg >> Radius;
     else if (token == "CAPACITY") *AC_cfg >> Capacity;
     else if (token == "CONTENTS") {
-        if (Contents == 0.0) *AC_cfg >> Contents;
+        if (Contents == -1.0) *AC_cfg >> Contents;
     } else cerr << "Unknown identifier: " << token << " in tank definition." << endl;
   }