diff --git a/core/src/bmi.jl b/core/src/bmi.jl
index c02b27278..0da32897d 100644
--- a/core/src/bmi.jl
+++ b/core/src/bmi.jl
@@ -10,24 +10,27 @@ BMI.initialize(T::Type{Model}, config_path::AbstractString)::Model = Model(confi
 
 Write all results to the configured files.
 """
-BMI.finalize(model::Model)::Model = write_results(model)
+function BMI.finalize(model::Model)::Nothing
+    write_results(model)
+    return nothing
+end
 
-function BMI.update(model::Model)::Model
+function BMI.update(model::Model)::Nothing
     step!(model.integrator)
-    return model
+    return nothing
 end
 
-function BMI.update_until(model::Model, time::Float64)::Model
+function BMI.update_until(model::Model, time::Float64)::Nothing
     (; t) = model.integrator
     dt = time - t
     if dt < 0
         error("The model has already passed the given timestamp.")
     elseif dt == 0
-        return model
+        return nothing
     else
         step!(model, dt)
     end
-    return model
+    return nothing
 end
 
 function BMI.get_value_ptr(model::Model, name::AbstractString)::AbstractVector{Float64}
@@ -55,8 +58,12 @@ function BMI.get_value_ptr(model::Model, name::AbstractString)::AbstractVector{F
     end
 end
 
-BMI.get_current_time(model::Model) = model.integrator.t
-BMI.get_start_time(model::Model) = 0.0
-BMI.get_end_time(model::Model) = seconds_since(model.config.endtime, model.config.starttime)
-BMI.get_time_units(model::Model) = "s"
-BMI.get_time_step(model::Model) = get_proposed_dt(model.integrator)
+BMI.get_current_time(model::Model)::Float64 = model.integrator.t
+BMI.get_start_time(model::Model)::Float64 = 0.0
+BMI.get_time_step(model::Model)::Float64 = get_proposed_dt(model.integrator)
+
+function BMI.get_end_time(model::Model)::Float64
+    seconds_since(model.config.endtime, model.config.starttime)
+end
+
+BMI.get_time_units(model::Model)::String = "s"