diff --git a/src/chemotaxis/celani.jl b/src/chemotaxis/celani.jl
index d772e91..125f4e3 100644
--- a/src/chemotaxis/celani.jl
+++ b/src/chemotaxis/celani.jl
@@ -60,7 +60,7 @@ end
 # to initialize the markovian variables at steady state
 # depending on the concentration field at the agent position
 function Agents.add_agent!(
-    pos::Agents.ValidPos,
+    pos::Any,
     A::Type{<:Celani{D}},
     model::AgentBasedModel,
     properties...;
@@ -74,7 +74,7 @@ function Agents.add_agent!(
 end
 
 function Agents.add_agent!(
-    pos::Agents.ValidPos,
+    pos::Any,
     A::Type{Celani{D,N}},
     model::AgentBasedModel,
     properties...;
diff --git a/src/model.jl b/src/model.jl
index 3bb4e24..bcf50ab 100644
--- a/src/model.jl
+++ b/src/model.jl
@@ -56,7 +56,7 @@ end
 
 
 function Agents.add_agent!(
-    pos::Agents.ValidPos,
+    pos::Any,
     A::Type{<:AbstractMicrobe{D}},
     model::AgentBasedModel,
     properties...;
@@ -84,7 +84,7 @@ otherwise default values from the constructor will be used.
 If unspecified, a random velocity vector and a random speed are generated.
 """
 function Agents.add_agent!(
-    pos::Agents.ValidPos,
+    pos::Any,
     A::Type{<:AbstractMicrobe{D,N}},
     model::AgentBasedModel,
     properties...;