diff --git a/sjplayer/src/sjplayer/AbstractScheduledController.d b/sjplayer/src/sjplayer/AbstractScheduledController.d
index 9949565..369beb5 100644
--- a/sjplayer/src/sjplayer/AbstractScheduledController.d
+++ b/sjplayer/src/sjplayer/AbstractScheduledController.d
@@ -4,12 +4,14 @@ module sjplayer.AbstractScheduledController;
 import std.algorithm,
        std.array,
        std.exception,
+       std.format,
        std.range.primitives,
        std.typecons;
 
 import sjscript;
 
-import sjplayer.ScheduledControllerInterface,
+import sjplayer.ScriptRuntimeException,
+       sjplayer.ScheduledControllerInterface,
        sjplayer.VarStoreInterface,
        sjplayer.util.Parameter,
        sjplayer.util.Period;
@@ -34,23 +36,31 @@ abstract class AbstractScheduledController : ScheduledControllerInterface {
    public:
     float opIndex(string name) const {
       if (!time_.isNull && name == "time") return time_.get;
-      return this_.GetVariable(name);
+      const temp = this_.GetVariable(name);
+      if (!temp.isNull) return temp.get;
+
+      throw new ScriptRuntimeException(
+          "unknown variable `%s`".format(name), srcline_, srcchar_);
     }
    private:
     AbstractScheduledController this_;
     Nullable!float              time_;
+
+    size_t srcline_, srcchar_;
   }
 
   void PrepareOperation(ref in ParametersBlock params) {
     user_vars_.clear();
 
-    auto vars = VarStore(this);
+    auto vars = VarStore(
+        this, Nullable!float.init, params.pos.stline, params.pos.stchar);
     params.parameters.
       filter!(x => x.type == ParameterType.OnceAssign).
       each  !(x => SetParameter(x, vars));
   }
   void ProcessOperation(float time, ref in ParametersBlock params) {
-    auto vars = VarStore(this, time.nullable);
+    auto vars = VarStore(
+        this, time.nullable, params.pos.stline, params.pos.stchar);
     params.parameters.
       filter!(x => x.type != ParameterType.OnceAssign).
       each  !(x => SetParameter(x, vars));
@@ -58,13 +68,21 @@ abstract class AbstractScheduledController : ScheduledControllerInterface {
   void FinalizeOperation(ref in ParametersBlock params) {
   }
 
-  float GetVariable(string name) const {
-    if (name in user_vars_) return user_vars_[name];
-    return varstore_[name];
+  Nullable!float GetVariable(string name) const {
+    if (name in user_vars_) {
+      return Nullable!float(user_vars_[name]);
+    }
+    auto temp = varstore_[name];
+    if (!temp.isNull) return temp;
+    // TODO: std constants
+    return Nullable!float.init;
   }
   void SetParameter(ref in Parameter param, ref in VarStore vars) {
-    (param.name.length >= 2 && param.name[0..2] == "__").
-      enforce("user defined variables must be prefixed '__'");
+    if (param.name.length < 2 || param.name[0..2] != "__") {
+      throw new ScriptRuntimeException(
+          "user defined variables must be prefixed as '__'",
+          param.pos.stline, param.pos.stchar);
+    }
     user_vars_[param.name] = 0;
     param.CalculateParameter(user_vars_[param.name], vars);
   }
@@ -114,7 +132,11 @@ ParametersBlock[] SortParametersBlock(R)(R params)
 
   auto before = Period(-1, 0);
   foreach (param; result) {
-    (!param.period.IsPeriodIntersectedToPeriod(before)).enforce();
+    if (param.period.IsPeriodIntersectedToPeriod(before)) {
+      throw new ScriptRuntimeException(
+          "the period is duplicated",
+          param.pos.stline, param.pos.stchar);
+    }
   }
   return result;
 }
diff --git a/sjplayer/src/sjplayer/ScheduledController.d b/sjplayer/src/sjplayer/ScheduledController.d
index 808723d..997b496 100644
--- a/sjplayer/src/sjplayer/ScheduledController.d
+++ b/sjplayer/src/sjplayer/ScheduledController.d
@@ -64,17 +64,17 @@ class ScheduledController(
     }
   }
 
-  override float GetVariable(string name) const {
+  override Nullable!float GetVariable(string name) const {
     switch (name) {
       static foreach (map_name, code; ParameterNameMap) {
         case map_name:
-          return mixin("target_."~code);
+          return Nullable!float(mixin("target_."~code));
       }
       default:
     }
     static if (MatrixModificationAvailable) {
       const value = matrix_factory_.GetValueByName(name);
-      if (!value.isNull) return value.get;
+      if (!value.isNull) return value;
     }
     return super.GetVariable(name);
   }
diff --git a/sjplayer/src/sjplayer/ScriptRuntimeException.d b/sjplayer/src/sjplayer/ScriptRuntimeException.d
new file mode 100644
index 0000000..0eaf3a7
--- /dev/null
+++ b/sjplayer/src/sjplayer/ScriptRuntimeException.d
@@ -0,0 +1,20 @@
+/// License: MIT
+module sjplayer.ScriptRuntimeException;
+
+import sjscript;
+
+///
+class ScriptRuntimeException : Exception {
+ public:
+  ///
+  this(
+      string msg, size_t srcline, size_t srcchar,
+      string file = __FILE__, size_t line = __LINE__) {
+    super(msg, file, line);
+    this.srcline = srcline;
+    this.srcchar = srcchar;
+  }
+
+  ///
+  size_t srcline, srcchar;
+}
diff --git a/sjplayer/src/sjplayer/VarStore.d b/sjplayer/src/sjplayer/VarStore.d
index e583132..24d69b6 100644
--- a/sjplayer/src/sjplayer/VarStore.d
+++ b/sjplayer/src/sjplayer/VarStore.d
@@ -2,7 +2,8 @@
 module sjplayer.VarStore;
 
 import std.exception,
-       std.format;
+       std.format,
+       std.typecons;
 
 import sjplayer.Actor,
        sjplayer.VarStoreInterface;
@@ -15,12 +16,12 @@ class VarStore : VarStoreInterface {
     actor_ = actor;
   }
 
-  override float opIndex(string name) const {
+  override Nullable!float opIndex(string name) const {
     switch (name) {
-      case "actor_x": return actor_.pos.x;
-      case "actor_y": return actor_.pos.y;
+      case "actor_x": return Nullable!float(actor_.pos.x);
+      case "actor_y": return Nullable!float(actor_.pos.y);
 
-      default: throw new Exception("unknown variable %s".format(name));
+      default: return Nullable!float.init;
     }
   }
 
diff --git a/sjplayer/src/sjplayer/VarStoreInterface.d b/sjplayer/src/sjplayer/VarStoreInterface.d
index cb497d4..b38e187 100644
--- a/sjplayer/src/sjplayer/VarStoreInterface.d
+++ b/sjplayer/src/sjplayer/VarStoreInterface.d
@@ -1,11 +1,13 @@
 /// License: MIT
 module sjplayer.VarStoreInterface;
 
+import std.typecons;
+
 import sjscript;
 
 ///
 interface VarStoreInterface {
  public:
   ///
-  float opIndex(string name) const;
+  Nullable!float opIndex(string name) const;
 }