diff --git a/gclc/src/main/java/net/bigeon/gclc/command/CommandParameters.java b/gclc/src/main/java/net/bigeon/gclc/command/CommandParameters.java
index 04dead6..bb93c59 100644
--- a/gclc/src/main/java/net/bigeon/gclc/command/CommandParameters.java
+++ b/gclc/src/main/java/net/bigeon/gclc/command/CommandParameters.java
@@ -75,7 +75,8 @@ public final class CommandParameters {
         for (final String string : bools) {
             booleanArguments.put(string, Boolean.FALSE);
         }
-        stringArguments = new ConcurrentHashMap<>(strings.size());
+        // Cannot use concurrent because of the null values.
+        stringArguments = new HashMap<>(strings.size());
         for (final String string : strings) {
             stringArguments.put(string, null);
         }
diff --git a/gclc/src/main/java/net/bigeon/gclc/command/ParametrizedCommandData.java b/gclc/src/main/java/net/bigeon/gclc/command/ParametrizedCommandData.java
index 8cedf83..91f4f27 100644
--- a/gclc/src/main/java/net/bigeon/gclc/command/ParametrizedCommandData.java
+++ b/gclc/src/main/java/net/bigeon/gclc/command/ParametrizedCommandData.java
@@ -87,6 +87,7 @@ import net.bigeon.gclc.manager.ConsoleInput;
 import net.bigeon.gclc.manager.EmptyInput;
 
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 /** An object to handle standardized command parameters.
  *
@@ -96,9 +97,9 @@ public final class ParametrizedCommandData {
     /** The boolean parameters mandatory status. */
     private final Set<String>          boolParams   = new HashSet<>();
     /** The string parameters mandatory status. */
-    private final Map<String, Boolean> stringParams = new HashMap<>();
+    private final Map<String, Boolean> stringParams = new ConcurrentHashMap<>();
     /** The parameters mandatory status. */
-    private final Map<String, Boolean> params       = new HashMap<>();
+    private final Map<String, Boolean> params       = new ConcurrentHashMap<>();
     /** The restriction of provided parameters on execution to declared paramters in
      * the status maps. */
     private final boolean              strict;
diff --git a/gclc/src/main/java/net/bigeon/gclc/utils/ReadingRunnable.java b/gclc/src/main/java/net/bigeon/gclc/utils/ReadingRunnable.java
index c1f81d6..2bef78d 100644
--- a/gclc/src/main/java/net/bigeon/gclc/utils/ReadingRunnable.java
+++ b/gclc/src/main/java/net/bigeon/gclc/utils/ReadingRunnable.java
@@ -78,6 +78,7 @@ import java.util.ArrayDeque;
 import java.util.Deque;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
@@ -165,7 +166,7 @@ public final class ReadingRunnable implements Runnable {
     /** The waiting status for a message. */
     private boolean waiting;
     /** The blocker for a given message. */
-    private final Map<String, Object> messageBlocker = new HashMap<>();
+    private final Map<String, Object> messageBlocker = new ConcurrentHashMap<>();
     /** The lock. */
     private final Object messageBlockerLock = new Object();
     /** The message being delivered. */